A lot of players try to use too much long arm (trying to be a mach man and use shoulder muscles) or try to use too much wrist movement.
I might be the odd ball out here who uses only a minor amount of wrist on serves to generate really heavy short underspin when I want. I believe that when someone tries to use too much wrist, they are messing up the timing and after all, the ball is in contact with the bat a short time and you cannot use all that wrist motion to accelerate the bat anywayz.
You accelerate the bat by moving your entire arm, but when arm is getting close to impact zone, you slow down or stop upper arm... if you are loose and do that, then the lower arm automatically receives the kinetic energy and moves forward on its own... at the right moment after this happens, you add to this with an impulse (helps to have a Popeye Bicep) and do a quick twitch to move lower arm more and at the end, wrist is still loose, and you use your Popeye forearm to snap that 10 degrees or so of your wrist right at the end... you are grabbing the ball and shooting it out. Aim for center of bat, trying for the tip is too difficult inconsistent.
This is the whipping motion Brett pimps up and it is a good pimp job he does... and he clearly articulates it.
A lot of us mach males try to add the power in the wrong way at the wrong time...take Ur vitamins and listen to TTD.