The Smart Investors

The 90% rule is common in stocks and it's applied by traders to deal with risk because they acknowledge that the largest portion of a move typically happened within the first 10%, leaving the rest to be susceptible to a reversal.

This 90% rule is a reminder to traders that most of a stock's action actually happens very quickly, within 10% of the beginning of the trend. So timing and discipline are everything needed. And if one chases the move afterwards, it can lead to reversals and unnecessary risk.