The bending of a joint requires the use of two muscles and not one muscle in one direction and another muscles in the other direction. One muscle to expand, while one muscle contracts.
You got some bad info and people wonder why I have a hard time with "trained" instructors. Oh wait, maybe is was the easiest way to teach, even though not accurate in how describing a joint bends.
While the bicep is contracting, the triceps is expanding. When the triceps is contracting, the biceps is expanding.
It is this transition the muscles do when changing the direction of the stroke that may require some people to pause and some not. Some can make this transitions smoothly, some can not.
Are you a physiologist? If so, this is why I have a problem with "trained" professors. Oh wait... maybe that was the easiest way to teach you? Lol
New flash: Muscles do NOT "expand" to move joints.
Muscles can only contract and relax. It's an electrochemical thing, caused by the action potential in the muscle fibers themselves. That's why the good Lord put them in opposition in every moving part of your body. Not just your body, but in the skeletons of just about every moving creature on the planet.
The best reason I can think of for the pause is to allow the triceps to totally relax so that it doesn't pull the lower arm off the intended line of aim when the biceps contracts to deliver the stroke.
This is one of the guiding principles of internal martial arts such as Tai Chi. The forms contain both quick and extremely slow movements, with small, almost imperceptible, pauses at the transitions between moves. This micro pause trains the body to relax the adductor muscles before the contraction of the abductors begins, and vise versa. Over time, great speed, power, and accuracy in blow placement is obtainable, while to the uninitiated, some old Chinese guy in the park is just "dancing in slow motion." Trust me, don't test him.
After thinking about this and watching dozens of top level players over the last several days, I have come to believe that almost all good players pause at the very end of the backstroke for at least a very brief - but still visibly detectable - time period.
Even the few where I saw absolutely no pause at all (Busta and Efren) might have benefited from it as well... but why fix what ain't broken, ya know?
EDIT:
Sorry, I reread my post and realized I got that wrong about the triceps relaxing at the pause, it is the biceps that is relaxing at that point. Haven't had the morning cuppa yet.
