I don't know. Why would body rotation slow down your arm and shoulder muscles?
If you accelerate your arm INTO a rotation of the body, shouldn't the rotation speed add to the total speed?
I'd agree with that. I think that what's going on here is the ever-elusive idea of always strike from your center... otherwise known as power comes from the earth/ground... or punching power comes from your hips. Whatever, they all end up meaning the same thing to me, in principle.
It's possible to strike solidly with just a flailing arm motion. Anyone can learn to deliver a backfist that's good enough to bust open a mouse over someone's eye, break a nose, or knockout a tooth sometimes with no body involvement, but with good technique in the shoulder, upper arm, forearm, wrist nd hand itself. It seems alien, but I can conceptually "see" a straight punch doing the same thing.
But, add in the additional energy which can be generated and delivreed through said strike with the additional muscle mass driving the body's weight/energy/momentum right into the same strike, exponential growth of energy delivery results.
Stand still, use only the shoulder, arm and hand to execute a simple hook punch. Watch how much the bag moves, even if you're giving it all you got with just that much.
Then, go ahead and throw the same punch with what we call "proper" body mechanics driving the punch. Viola! Bag ends up giving you a much better feedback by flying away.a