Most of the classic patterns have a bias, ie people read in books that they are supposed to go up or go down. For example a descending triangle is supposed to go down while a cup and handle should go up.

These same patterns usually have a trendline or support-resistance line or high or low that negates the pattern. In a descending triangle, you can join the descending highs into a straight line. The pattern failure trade signals when price breaks through that line. The basic TA types that saw the pattern in a book or read about it at a web site see the pattern start to fail and they jump ship. You're positioned the other way and get to ride their panic/fear/dumbness.