NEW YORK (TheStreet) -- The "Janet Yellen stock rally" Friday proved to be a "sell on strength" opportunity for investors. "Death cross" formations are spreading around the world in major stock market indices. 

A "death cross" occurs when the 50-day simple moving average declines below the 200-day simple moving average. This formation is a warning of further declines ahead.

So when markets become dicey, what do investors do? They turn to the safety of gold, gold-related instruments and U.S. Treasury notes and bonds.

As a result of Yellen's comments in a speech last week, the U.S. dollar improved, as did the price of crude oil, which is priced in dollars. Here's how to trade these markets using these four exchange-traded funds.

The 20+ Year Treasury Bond ETF (TLT) - Get Report stayed above its 200-week simple moving average. The SPDR Gold Shares ETF (GLD) - Get Report ended the week with a positive weekly chart. The iShares GSCI Commodity-Index Trust Fund (GSG) - Get Report stayed negative but oversold. The Deutsche Bank USD Index (UUP) - Get Report  became neutral on dollar strength on Friday.

Here's the weekly chart for the bond ETF.


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The bond ETF, which is a basket of U.S. Treasury bonds with maturities of 20+ years to 30 years, had a close of $121.54 on Friday, down 3.5% year to date with the S&P 500I:GSPC down 6.2%.

The bond ETF is just below its 50-day simple moving average of $122.10 and is below its 200-day simple moving average of $124.83. Keep in mind that this ETF traded as high as $128.92 on the spike higher on "Black Monday," Aug. 24.

The weekly chart remains negative as the ETF ended last week just below its key weekly moving average of $122.09, with the 200-week simple moving average of $117.55, which is considered the uptrend for bonds. The weekly momentum reading declined to 56.18 down from 60.71 on Sept. 18.

Investors looking to buy the bond ETF should place a good till canceled limit order to buy the ETF if it drops to the 200-week simple moving average of $117.55.

Investors looking to reduce holdings should place a good until canceled limit order to sell the ETF if it rises to $132.13, which is a key level on technical charts until the end of 2015.

Here's the weekly chart for the gold exchange-traded fund.


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The gold ETF, which is backed by gold bullion, had a close of $109.81 on Friday down 3.3% year to date. This ETF is above its 50-day simple moving average of $107.02 and below its 200-day simple moving average of $113.20. Keep in mind that this ETF traded as high as $112.12 on the spike higher on Aug. 24.

The weekly chart is positive, with the ETF above its key weekly moving average of $108.61, but well below its 200-week simple moving average of $135.34. The weekly momentum rose to 41.29 last week up from 36.15 on Sept. 18.

Investors looking to buy the gold ETF should place a good till canceled limit order to buy the ETF if it drops to $106.24, which is on the uptrend shown on the weekly chart.

Investors looking to reduce holdings should place a good until canceled limit order to sell the ETF if it rises to $114.17, which is a key level on technical charts until the end of 2015.

Here's the weekly chart for the commodity-index ETF.


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The commodity ETF, which is 70% to 75% weighed to energy and crude oil, had a close of $17.18 on Friday, down 20.4% year to date. This ETF is above its "Black Monday" low of $15.90.

The weekly chart shifts to positive given a close this week on Oct. 2 above its key weekly moving average of $17.64. The weekly momentum reading rose to 21.95 last week up from 20.01 on Sept. 18, now above the oversold threshold of 20.00. The 200-week simple moving average is declining at $29.73.

Investors looking to buy the commodities ETF should place a good till canceled limit order to buy the ETF if it drops to $16.07, which is a key level on technical charts until the end of this week.

Investors looking to reduce holdings should place a good until canceled limit order to sell the ETF if it rises to $22.98, which is a key level on technical charts until the end of 2015.

Here's the weekly chart for the dollar index ETF.


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The dollar ETF, which is basket of currencies which includes the euro, Japanese yen, British pound, Canadian dollar, Swedish krona and Swiss franc, had a close of $25.08 on Friday up 4.6% year to date. The ETF is just below its 50-day and 200-day simple moving averages of $25.17 and $25.09, respectively, and above its "Black Monday" low of $24.20.

The weekly chart is neutral, with the ETF just above its key weekly moving average of $25.06. The weekly momentum reading declined to 52.03 last week down from 53.78 on Sept. 18. The dollar ETF is well above its 200-week simple moving average of $22.72.

Investors looking to buy the dollar ETF should place a good till canceled limit order to buy the ETF if it drops to $24.59, which is a key level on technical charts until the end of 2015.

Investors looking to reduce holdings should place a good until canceled limit order to sell the ETF if it rises to $40.88, which is a key level on technical charts until the end of 2015.

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