Anatomically speaking, muscles are made up of long, thin fibers of muscle. And these fibers are composed of single muscle cells strung together end to end. A huge number of muscle fibers must be bundled together to form any muscle group.
Physiologically, the mind sends nerve impulses to the muscle group that will be contracted to move a weight in an exercise. The contraction of one muscle cell in a fiber causes the entire fiber to shorten in length, and the contraction of two cells in a fiber causes it to shorten even more. Overall, the number of muscle cells in a muscle fiber that actually shorten to contract a muscle group depends on the strength of the nerve impulses sent by the brain to the individual cells. The stronger this nerve impulse, the greater the number of cells caused to shorten. And the more cells you can shorten like this, the stronger and more complete the contraction of a full muscle mass.
In using this principle, you will utilize exercises in which you have a full weight load on the working muscles when they are completely contracted. Ordinarily, most bodybuilding movements done on machines provide a peak contraction effect.
At the top of each peak contraction movement, you should try to hold the weight for several seconds with your fully contracted muscles, building as much tension as possible into the working muscle groups. Holding the weight for a slow count of three at the top of the leg extension movement, for example, allows you to contract your quadriceps much harder, thereby adding mass, quality, and deepcuts to your front thighs.
Those advanced lifters will know what weight to use without injury but if you are not an advanced lifter or have never used this technique before, use caution and a weight a little lighter than you normally use, as it will definitely cause you muscles to be very sore the next day if performed correctly and the proper amount of weight is used. Over time and with experience, you will come to know your body and what weight will cause the desired effect.
This is not an easy way to train and it is not to be used all the time, as with all techniques you can cycle it in when you feel you need a shock or a change is needed to keep your muscles guessing and growing!