During the 1960s and 1970s, a pec bounce was a power move common in bodybuilding competition and in movies to establish that a character was strong and muscular. Since then, the move has become perceived as cheesy -- as likely to be a parody than a serious expression of strength. Still, it can be a fun and noticeable way to emphasize your developing chest muscles.
Begin or continue a weight training program that builds your pecs. This can be part of a full-body regimen, or a program that emphasizes the four sections of your chest: upper, lower, inner and outer. Examples of chest exercises include machine and dumbbell flies, bench press, pushups and chest press.
Pay attention to how the pectoral muscles feel when they are flexed during your chest workouts. Isolating this flexion is how you'll learn to do a pec bounce.
Look in a mirror while flexing just your pectoral muscles. In your first attempts, you might find that you also flex your arms, neck or shoulder muscles. Continue practicing while you look in the mirror, focusing on isolating your pectorals until you can see your pecs flex by themselves.
Practice flexing one pectoral muscle at a time once you've successfully isolated your pecs as a group. Continue this practice until you can reliably do so.
Flex one pectoral muscle explosively -- that is, in a sudden and hard contraction. This will give the muscle contraction the twitching bounce that gives this move its name. As that pec relaxes, flex the other pectoral in the same way.
Continue practicing your pec bounces. Like any other skill, it will take more than one practice session to be able to perform this move reliably.