Essentially this concept is like a stone being dropped in a pool creating ripples through out. The size of the stone is dependent upon the degree of your muscular contraction.
The harder you squeeze, the bigger the stone and larger the ripples, which in turn radiate out and activate more motor units for the movement. This results in more power and is the first step to unleashing 290x MORE GH in your body after just one set.
Let's do a test. Squeeze your fist just a little bit and you will notice the contraction in your hand mostly. Now squeeze a bit harder and you will notice it running up your arm perhaps to your shoulder.
Now squeeze as hard as you possibly can you will feel your midsection, glutes and hopefully the rest of your body participating in the contraction. The whole is greater than the sum of its parts.
You will notice yourself able to create more power and open a gateway of anabolic hormones in any activity by using the 3 parts to this principle.