The 10:15 rule is basically a belief that the share imbalance that builds up when people put in trades before the market opens takes until around 10:15 to balance out. Because of this imbalance, stocks are often pushed in one direction or another. Professionals often trade against this direction when they sense that the pressure has dissipated.
In my experience, this trend does have some merit. However, the time seems to vary from anywhere between 10:00 and 11:00. I would also state that the high/low may be a short term high/low. But those extremes present great entry points if you know how to read them. A lot of times the stock will retest those levels. If the stock breaks through, it will likely retest and provide a great opportunity to enter. If they bounce off those levels and show a divergence in the indicators (I prefer RSI), they provide a great opportunity to enter and have a fairly tight stop loss.
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