In kendo, when we initiate an attack and approach the opponent during seme-ai, we are advised by wise senseis to not only maintain a relaxed grip on the shine, but to relax it even more as we apply pressure (metaphorically, speaking, of course, not really in terms of force per unit area) on the opponent. This is done to preemptively compensate for our natural tendency to tighten up at the beginning of the actual physical attack.
Tightening up is a natural reaction of the human body to physical stress. In fact, by consciously flexing our gip on a dumbbell, for example, we are able to pull off a couple of additional biceps curls beyond what would other wise be our maximum number of reps. This very effective principle, among others, is taught by Pavel Tsatsulin, who use to train special forces operators in various countries. According to him, strength requires tension, while flexibility requires relaxation. Ability to switch from one to another quickly and at will is the key to high physical performance.
