Thank you - no one’s ever explained it that way.
But what about the third base coach flapping his arms and legs around... Who is he signalling and how does it help
He is usually directing the runner to hold up or sending them home using their judgement as to where the ball was hit and who fielded it as well as the speed of the runner. This allows the runner to just concentrate on running and circle the bases as fast as possible.
The runner doesn’t watch the ball, the coach does and tells the runner whether to hold or continue. The exaggerated windmill is so there is no mistaking the signal.