I was doing some backtesting (don't bother commenting/arguing if you don't trade with patterns or backtesting) and found that on "Rollover" day 70% off the time the market drops at the open for about an hour and a half, then pops back up until about noon. It's usually 4 points on the fall down, and 4-5 points on the rise back up.
Pointing out a tendency, not a bullet-proof strategy or system.