NEW YORK (TheStreet) -- The I:VIX volatility index just triggered a rare buy signal, meaning now may be a good time to invest in VIX-tracking ETFs for big gains. To fully understand how this works, allow me to go over a few basics.

The CBOE Volatility Index, or VIX, measures the expected 30-day volatility for the S&P 500. The VIX at 14 implies a 14% move (up or down) over the next 12 months, or 4.04% over the next 30 days, or 0.88% daily.

But the VIX is not the only volatility measure. There is also the CBOE 3-month Volatility Index VXV, which measures the expected 90-day volatility for the S&P 500.

The VXV trades currently around 17, which implies daily volatility of 1.07%.

At the moment, volatility traders are less concerned about short-term volatility (expected 30-day volatility is 0.88%) than longer-term volatility (expected 90-day volatility is 1.07%).

In simple terms, VIX traders are complacent. They would rather focus on potential far away speed bumps, and choose to ignore the risk for short-term speed bumps. Such complacency is a contrarian indicator.

But how can we measure the scope of this complacency?

The easiest and most effective way is to calculate and chart the ratio between the 30-day VIX and 90-day 'VIX,' or the VXV.

I've done exactly that and plotted the VIX:VXV ratio against the S&P 500 and VIX:

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On Monday, the VIX:VXV ratio dropped to the lowest level since April 2013. The dashed red lines highlight how similar readings affected the S&P 500 and VIX.

Although the scope varied, the S&P 500 declined every time, and the VIX rallied every time (at least short-term) the VIX:VXV ratio dropped below 0.79.

What makes this signal even more interesting is the green trend line support for the VIX. A close below 12.70 would constitute a breach of support, and postpone the VIX buy signal as long as it stays below 12.70.

None if this data helps determine how high the VIX (or how low the S&P 500) may go, but the green trend line provides are clear point of ruin (stop-loss).

Although end of quarter balancing has caused some nasty late-March selloffs in the past, I don't think there will be a huge move this year. A look at VIX seasonality (VIX seasonality chart available here) will also help to put this VIX trade into perspective.

Nevertheless, a 10% or 20% VIX spike could translate into decent gains for VIX ETFs like the iPath S&P 500 VIX Futures ETF (VXX) - Get iPath Series B S&P 500 VIX Short-Term Futures ETN Report or the leveraged VelocityShares Daily 2x VIX ETF (TVIX) - Get VelocityShares Daily 2x VIX Short-Term ETN Report.

Since a VIX spike may be short-lived, gains should probably be taken quickly.

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