Plyometrics are movements that teach the body to recruit the maximal amount of force in the shortest amount of time. Sounds like the definition of power, right? Used appropriately, plyometric training is the key to transferring the strength build in the weight room to speed in the competitive environment. They will help move the force-velocity curve up and to the right. Properly implemented into your training program, plyometrics will make you faster.
Try this experiment. Stand next to a wall with your hands above your head and jump as high as you can. Note how high you got. Now move away from the wall before you do a second attempt. This time, take a couple of steps toward the wall and then quickly drop into a partial squat before exploding up off the ground. You jumped higher this time, didn't you? What you just experienced is a live demonstration of the Stretch-Shortening Cycle--the underlying principle of plyometric training.
One of the mechanisms by which the body conserves energy to move more efficiently, the stretch-shortening cycle is an active stretching of a muscle under load (eccentric contraction) followed by an immediate shortening of the muscle (concentric contraction). The time between the start of the eccentric movement and the concentric movement is called the amortization phase. During this period, elastic energy is stored in the muscle tendon unit. If the transition between eccentric loading and concentric movement takes too long, this elastic energy is dissipated as heat. However, if this transition occurs rapidly enough, the stored energy is utilized to make the subsequent movement not only more efficient but more powerful as well.
The last time you had a physical, you probably had the doctor tap your patella tendon with a small rubber hammer. And if he hit the right spot the right way, you had an involuntary contraction which would've made him a soprano if he had been standing in front of you without wearing a cup. You had no control over your leg, so you'd get to take no credit for his new singing career. That was the stretch reflex, or myotatic reflex, in action. And that's the basis of plyometrics.