Fist clinching is the sudden tightening of the fist just before impact with the target. Early boxers, such as Jack Dempsey and Bob Fitzsimmons, were proponents of this type of punching. For speed of arm movement, the fist should remain relaxed and not clinched until just before it reaches its target.
To help with punching speed and quickly movements, when the arms are held in the guard position, the fists should be half-closed and relaxed. When a punch is fired, the fist stays relaxed and loose until just before the moment of impact, at which point the fist is suddenly clinched very tightly and the arm, shoulder, and body are tensed. This movement suddenly locks the body from fist-to-foot and permits the maximum transfer of power into the target.
When tensing the body to resist the reaction force in a punch, body muscles are tensed to lock the body in position so that the target absorbs the impact rather than the person punching. We all know the fist should be clinched at impact, but when should it initially be clinched?
Beginners should probably clench the fist all the time. Intermediate students should begin to try to clench the fist only at impact. High ranking students should have the timing and distancing necessary to choose the proper moment to clench the fist, so that safety is not a concern and they may benefit from the total relaxation provided by a loose hand.
During sparring drills, many high ranks like to keep their fists loose even on impact, so they may strike and actually touch the opponent without injuring them. After long years of training, the expert may easily choose when to clench the fist or to keep it relaxed.