All Eyes Are On One Particular S&P 500 Stat

Thursday's 2% drop in the S&P 500 put the index at 1,928, down 4.5% from its September 19 high of 2,019.

This is one of the more notable pullbacks we've seen in what has been a very low-volatile rally in the stock market. In fact, the S&P 500 hasn't closed below its 200-day moving average (DMA) since November 20, 2012.

"All eyes will be on the 200 DMA for the SPX today," MKM Partners' Jonathan Krinsky writes. "That currently comes in at 1905, which is also the area of the August lows."

UBS's Art Cashin just warned that if the S&P broke though 1,925, we could tumble all the way to 1,900.

"We continue to expect that the SPX Cash will test its 200 DMA before this correction is over," Krinsky writes. "At that point we can evaluate the structure to determine of a bigger pullback is at hand."

Krinsky notes that the Equal Weight S&P 500 actually closed below its 200-DMA for the first time since November 2012.