Corporate insiders sell their own companies' stock for a number of reasons.

They might need the cash for a big personal purchase such as a new house or yacht, or they might need the cash to fund a charity. Sometimes they sell as part of a planned selling program that they have put in place for diversification purposes, which allows them to sell stock in stages instead of selling all at one price.

Other times they sell because they think their stock is overvalued and the risk/reward is no longer attractive. Some even dump their own stock because they have inside knowledge that a competitor is eating their lunch and stealing market share.

But insiders usually buy their own shares for one reason: They think the stock is a bargain and has tremendous upside.

The key word in that last statement is "think." Just because a corporate insider thinks his or her stock is going to trade higher, that doesn't mean it will play out that way. Insiders can have all the conviction in the world that their stock is a buy, but if the market doesn't agree with them, the stock could end up going nowhere. Also, I say "usually" because sometimes insiders are loaned money by the company to buy their own stock. Those loans are often sweetheart deals and shouldn't be viewed as organic insider buying.

Stocks with notable insider activity is something that I tweet about on a regular basis. These are also the exact type of stocks that I love to trade and alert in real-time.

At the end of the day, it's institutional money managers running big mutual funds and hedge funds that drive stock prices, not insiders. That said, many of these savvy stock operators will follow insider buying activity when they agree with the insider that the stock is undervalued and has upside potential. This is why it's so important to always be monitoring insider activity but twice as important to make sure the trend of the stock coincides with the insider buying.

Recently, a number of companies' corporate insiders have bought large amounts of stock. These insiders are finding some value in the market, which warrants a closer look at these stocks.

Pandora Media

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One broadcasting player that insiders are active in here is Pandora Media  (P) , which provides Internet music streaming services in North America. Insiders are buying this stock into notable strength, since shares have risen by 15.3% over the last three months.

Pandora Media has a market cap of $2.7 billion and an enterprise value of $2.5 billion. This stock trades at a reasonable valuation, with a price-to-sales of 2.12 and a price-to-book of 3.90. Its estimated growth rate for this year is -588.90%, and for next year it's pegged at 77.3%. This is a cash-rich company, since the total cash position on its balance sheet is $349.26 million and its total debt is $239.01 million.

An officer just bought 250,000 shares, or about $2.83 million worth of stock at $11.33 per share. That same officer also just bought 250,000 shares, or about $2.74 million worth of stock at $10.97 per share. From a technical perspective, Pandora Media is currently trending above its 50-day moving average and just below its 200-day moving average, which is neutral trendwise. This stock has been uptrending strong over the last two months, with shares moving higher off its low of $8.05 a share to its recent high of $11.85 a share. During that uptrend, this stock has been making mostly higher lows and higher highs, which is bullish technical price action. That move has now pushed shares of Pandora Media within range of triggering a big breakout trade.

If you're bullish on Pandora Media, then I would look for long-biased trades as long as this stock is trending above its 20-day moving average of $10.48 a share and then once it breaks out above some near-term overhead resistance levels at $11.85 a share to its 200-day moving average of $12.87 a share with high volume. Look for a sustained move or close above those levels with volume that hits near or above its three-month average volume of 9.08 million shares. If that breakout fires off soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $14 to $16.23 a share.

Western Digital

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A technology stock that insiders are jumping into here is Western Digital(WDC) - Get Report , which engages in the development, manufacture, sale and provision of data storage solutions. Insiders are buying this stock into weakness, since shares have dropped by 25.4% over the last six months.

Western Digital has a market cap of $13 billion and an enterprise value of $7 billion. This stock trades at a premium valuation, with trailing price-to-earnings of 13.1 and a forward price-to-earnings of 10.9. Its estimated growth rate for this year is -35.8%, and for next year it's pegged at -14.3%. This is a cash-rich company, since the total cash position on its balance sheet is $6.03 billion and its total debt is $2.20 billion. This stock currently sports a dividend yield of 4.4%.

The CEO just bought 22,500 shares, or about $1.02 million worth of stock, at $45.53 per share.

From a technical perspective, Western Digital is currently trending above its 50-day moving average and below its 200-day moving average, which is neutral trendwise. This stock has been uptrending strong over the last few weeks, with shares moving higher off its low of $34.99 a share to its recent high of $47.78 a share. During that uptrend, this stock has been consistently making higher lows and higher highs, which is bullish technical price action. That move has now pushed shares of Western Digital within range of triggering a near-term breakout trade.

If you're bullish on Western Digital then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $42.58 a share and then once it breaks out above some near-term overhead resistance levels at $48 to $49 a share and then above more key resistance at $51.15 a share with high volume. Look for a sustained move or close above those levels with volume that registers near or above its three-month average action of 6.60 million shares. If that breakout triggers soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at its 200-day moving average of $56.72 to $60 a share.

Tallgrass Energy Partners

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One energy player that insiders are jumping into here is Tallgrass Energy Partners (TEP) , which acquires, owns, develops and operates various midstream energy assets in North America. Insiders are buying this stock into big strength, since shares have ripped up 29.2% over the last three months.

Tallgrass Energy Partners has a market cap of $3.2 billion and an enterprise value of $4.4 billion. This stock trades at a reasonable valuation, with a trailing price-to-earnings of 25.3 and a forward price-to-earnings of 14.5. Its estimated growth rate for this year 17.8%, and for next year it's pegged at 38.7%. This is not a cash-rich company, since the total cash position on its balance sheet is $2.88 million and its total debt is $1.20 billion. This stock currently sports a dividend yield of 6.3%.

The CEO just bought 7,500 shares, or about $325,000 worth of stock, at $43.45 per share. That same CEO also just bought 5,300 shares, or about $239,000 worth of stock, at $45.11 per share.

From a technical perspective, Tallgrass Energy Partners is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock recently pulled back off its high of $50.78 a share to its near-term low of $43 a share, which was right above its 50-day moving average of $41.14 a share. This pullback is coming after a massive uptrend over the last four months, which saw shares of Tallgrass Energy Partners rise from $25.36 a share to that recent high of $50.78 a share.

If you're in the bull camp on Tallgrass Energy Partners, then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $41.14 a share or above its 200-day moving average of $39.10 a share and then once it breaks out above its 20-day moving average of $46.06 a share to some more near-term resistance around $48 a share with volume that hits near or above its three-month average action of 379,241 shares. If that breakout materializes soon, then this stock will set up to re-test or possibly take out its next major overhead resistance level at $50.78 a share. Any high-volume move above that level will then give this stock a chance to make a run at $55 to $60 a share.

Phillips 66

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Another energy player that insiders are loading up on here is Phillips 66(PSX) - Get Report , which operates as an energy manufacturing and logistics company. Insiders are buying this stock into notable weakness, since shares have slid lower by 12.2% over the last six months.

Phillips 66 has a market cap of $42.2 billion and an enterprise value of $49.3 billion. This stock trades at a cheap valuation, with a trailing price-to-earnings of 12 and a forward price-to-earnings of 11.2. Its estimated growth rate for this year is -30.4, and for next year it's pegged at 33.5%. This is not a cash-rich company, since the total cash position on its balance sheet is $1.72 billion and its total debt is $8.84 billion. This stock currently sports a dividend yield of 3.1%.

A beneficial owner just bought 642,273 shares, or about $51.22 million worth of stock, at $79.10 to $79.76 per share.

From a technical perspective, Phillips 66 is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been consolidating and trending sideways over the last month or so, with shares moving between $76.26 on the downside and $81.08 on the upside. Any high-volume move above the upper-end of its recent sideways trending chart pattern could trigger a big breakout trade for shares of Phillips 66.

If you're bullish in Phillips 66, then I would look for long-biased trades as long as this stock is trending above its 20-day moving average of $78.72 a share or above its range low of $76.26 a share and then once it breaks out above some near-term overhead resistance levels at $81.08 a share to its 200-day moving average of $81.49 a share and then above its 50-day moving average of $82.83 a share with high volume. Look for a sustained move or close above those levels with volume that hits near or above its three-month average action of 2.83 million shares. If that breakout kicks off soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $86 to $88, or even $90 a share.

National Instruments

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My final stock with some large insider buying is technology player National Instruments(NATI) - Get Report , which designs, manufactures, and sells systems to engineers and scientists worldwide. Insiders are buying this stock into modest weakness, since shares have fallen by 9% over the last six months.

National Instruments has a market cap of $3.6 billion and an enterprise value of $3.3 billion. This stock trades at a reasonable valuation, with a trailing price-to-earnings of 41.4 and a forward price-to-earnings of 27.2. Its estimated growth rate for this year is 9.5%, and for next year it's pegged at 29.6%. This is a cash-rich company, since the total cash position on its balance sheet is $328.61 million and its total debt is $25 million. This stock currently sports a dividend yield of 2.8%.

A director just bought 394,819 shares, or about $10.85 million worth of stock, at $27.50 per share.

From a technical perspective, National Instruments is currently trending above both its 50-day and 20-day moving averages, which is bullish. This stock has been uptrending over the last few weeks, with shares moving higher off its low of $26.31 a share to its recent high of $28.71 a share. During that uptrend, this stock has been consistently making higher lows and higher highs, which is bullish technical price action.

If you're bullish on National Instruments, then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $28 a share or above its 20-day moving average of $27.40 a share and then once it breaks out above some near-term overhead resistance levels at $29 to $29.25 a share with volume that hits near or above its three-month average action of 389,127 shares. If that breakout fires off soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $30 to $30.50, or even $31.25 to $32 a share.

Disclosure: This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.