A proactive trading strategy in which a trader takes profits by closing out of a short position or buying into a long position. This strategy is used when the price of the asset being traded is still falling but is expected to reverse and move against the trader. This is the opposite of "selling into strength".
For example, let's say that a trader believes that ABC stock will fall below $5 to $4.50 before rising above $5. Therefore, the trader would buy into the weakening stock price at a price below $5 and wait until the falling trend reverses and the price rises before selling and taking a profit. A short seller may also buy weakness by closing out his or her position. This would be done by buying into a falling stock with the anticipation that the stock price will soon reverse and start to rise.

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 greater room for error. As the saying goes, "missed money is better than lost money".