This year we are discussing the Book of Martial Power (BOMP)
"Heaviness," check. Got it. Next.
Ha, get it? That was a fat joke ;-)
No, seriously, Pearlman discusses in this chapter, an idea that he calls Heaviness. This is basically the ability to properly manage your structure so that you can relax, freeing your body mass to drop and affect the opponent.
Consider - strength is highly correlated with sheer body mass. That is, the heavier you are, the stronger you are (in general). This is largely because muscle is heavy. The more of it you have, the heavier you will be, and the stronger you will be.
We discussed, some months back the idea that we can only bring to bear some percent of our power at any given time. We would like to optimize our strength by eliminating the things that interfere with our ability to apply that strength to the other guy.
But in the same way, we can only drop some fraction of our mass (largely muscle) onto the enemy. The remainder of our (muscle) mass is involved in keeping ourselves from falling, and fixing our posture, and some of it is wasted as tension, etc...
But, to the degree we are able to minimize the amount of our mass (muscle) that is serving some other purpose, the more mass we can drop, like a brick (or even better, like a sack of water) onto the enemy. If we are using some of our muscle to keep the rest of our muscle from hitting the ground, then we have very little left to hammer the opponent with.