This is Dr. Mike Marshall's "pitching forearm flyout." I have shortened the name.
When the pitching elbow moves in a pronounced arc, inertia slings the forearm away from the body. To keep the olecranon of the ulna from slamming into the olecranon fossa of the humerus, the brachialis muscle eccentrically contracts.
Reciprocal inhibition prevents the triceps brachii from contracting while the brachialis is contracting. The brachialis must relax before the triceps can contract. When forearm flyout occurs, the triceps brachii does not contract, therefore it does not contribute to the pitch.