This stretch reflex happens when you jump, one reason we often refer to plyometrics as jump training. For example, if you jump up onto a box or step and then jump down, the quads stretch as your knees bend and then quickly contract again with the next jump. It's the prestretch of the first jump that enhances the second jump.
While plyometric training is something athletes use for training, the average exerciser can reap the benefits as well in the form of more power, more strength, more endurance and burning more calories.
There are some downsides to this type of training. It's easy to injure yourself with all that jumping, especially when you're jumping down from a high platform or step. Each time you land, your joints sustain about seven times more force than your body weight, so it's important to carefully consider the types of exercises you're doing and to ease into this type of training. A personal trainer or coach is a great resource for helping you set up a plyometric training program that fits your fitness level and goals.
Examples of Plyometric Exercises
More about plyometric training.
American Council on Exercise. ACE Personal Trainer Manual, 3rd Edition. San Diego: American Council on Exercise, 2003.