Given arm speed increases the velocity generated by the arm whipping action, pitchers often think of muscling the ball as generating arm speed. I think the key to generating more velocity is getting away from muscling with the arm, but instead focusing on a loose, but very whippy arm.
Well said. It is all about arm whip. Only thing I would add is that it is ultimately the speed of the fingers that determine velocity. Yes, you need to accelerate the arm but do not forget the wrist and positioning of the ball on the pads of the fingers. I get my students to focus on whipping the fingertips which introduces different nomenclature to better differentiate from what they may have been taught when learning HE.