A proactive trading strategy carried out by selling out of a long or into a short position when the price of the asset being traded is still rising but is expected to reverse in price. Opposite of "buying into weakness".
For example, say a trader believes ABC stock will rise above $5.00 but expects it to reverse at $5.75. If the trader buys ABC stock at $5.00 and sells when the price hits a predetermined exit price of $5.50, that trader would be selling into strength. Conversely, a short seller may sell into a rising price with the anticipation that the stock price will soon decline.

Many traders will wait for confirmation of a change in price movement before reacting. However, by the time a reversal is /confirm/ied, it may be too late and the trader may end up losing. Thus, by trading against the prevailing trend in the anticipation that it will soon reverse, the trader allows him- or herself a greater margin of safety. As the saying goes, "missed money is better than lost money".