Flinching, big and small muscles?

I was just watching the PGA on TV and, one of older teaching announcers, when analyzing a bad shot said, "It's the same old thing. When we get under pressure, the small muscles freeze up and the big muscles get more active."
Made me think about some of the bad moves we make in trap and why many go to release triggers.
But I am just starting to evaluate the it.
So, what do all you guru's think of that statement, whether true or not it is correct, and whether it relates to trapshooting.
Regards, Jake

 

It's the big muscle, the one between your ears.