W D GANN the basics..

Rule 7: 7 to 10 day rule for active or blow-off markets.

This rule works well in active markets. If market is trend up and the market trends up for 7 day to 10 days in a row, (again, trend up is making higher highs and higher lows each day), start following a sell stop under the 7th day up lows to liquidate or go short (or long). (To enter into a new short position, one needs apply all the other rules to make sure there are no conflicts.)

When's & How's of Pyramiding: