Karateka
The definition you quoted is absolutely correct. But read it very carefully. You cannot "load" a muscle very well by going down slowly. You do that best with "the downward force of your body" and "then immediately you must contract the muscle to push yourself back up." Going down slowly isn't very much downward force. Going down quickly is much more.
Chu talks about this
stretch-shortening cycle in his book "Jumping into plyometrics."
the muscles can briefly store the tension developed by rapid stretching so that they possess a sort of potential elastic energy. ... The stretch reflex is another mechanism integral to the stretch-shortening cycle. ... The stretch, or myostatic, reflex responds to the rate at which a muscle is stretched and is among the fastest in the human body.
I first came across the idea of the importance of the speed of the eccentric/concentric cycle in Kulund's book "The injured athlete." That reference states it isn't how far you go down that is important, but how
quickly you go down.
The definition isn't arbitrary. Stimulating the stretch reflex is the whole
raison d'etre of the exercise. Learning how to invoke and use it gives you power that is more than what you can get with a simple concentric movement (a simple muscle contraction).
The whole idea of
fajin is learning how to take maximum advantage of a rapid stretch-shortening cycle. Experienced athletes learn to cause these movements to happen in a wave from one end of the body to another.
Just watch a baseball batter swing a bat. The stretching starts low at the legs, moves to the hips and waist, and goes through the shoulders to the wrists. At each point in "the wave," you are invoking that
neuromuscular response. By making this happen from one end of the body to the next, you add to the power at each muscle group. By the time it hits the wrists, they become the crack on the end of the whip.
A martial artist can do the same with a good thrust.
Please understand that I am not "dissing" the exercise you suggested. Going down slowly and then exploding up is a very, very good STRENGTH exercise, and is exactly what you should be doing in the weight room. It helps to develop the MUSCLES through the full range of motion.
- Bill