The bounce is the use of the elastic rebound at the bottom of the squat or clean to recover from the bottom position more easily and with more speed. It is the combination of three elements: the literal bounce of the upper leg off the lower leg, the stretch-shortening reflex in the muscles of the legs and hips, and the elastic whip of the barbell.
This movement can be easily misinterpreted as relaxing under the bar, and is occasionally practiced as such by athletes who have learned it solely through observation and possess no understanding of its principles. This is a critical mistake for two reasons. First, anything even resembling relaxation under heavy loads is an opportunity for injury. Second, any relaxation will reduce the stability of the relative position of the hips and spine, meaning that downward force at the bottom of the squat will cause changes in these positions that will absorb some of that force; consequently, less of that force can be stored as elastic energy to then contribute to the recovery from the bottom.
This is a blow to performance and safety that can’t be afforded when handling significant loads. The athlete must remain tight and structurally sound throughout the movement, bracing for the abrupt arrest of downward movement and the subsequent rapid change of direction. This being said, the athlete must also allow the weight to push his or her body down rapidly as he or she nears the bottom in order to create the speed necessary for an optimal stretch reflex.
The bounce should generally be used in training with the clean and front squat to train the reflex and timing.