Plyometrics is the activity of rapid alternation of lengthening and shortening of specific muscle groups while resistance is continuously being applied to them. This lengthening and shortening cycle when performed in rapid succession allows the muscles to store some of the lost energy in the lengthening phase for use during the contraction phase.
The faster your muscles can perform this lengthening and shortening with resistance cycle, the more powerful your movements become. Strength + Minimal Time Factor = Power. That's the science of Plyometrics.