Ok, and surely more than my , 2 cents. I have a big, powerfull serve, not sure how fast it is. However, the real keys I work on when it is off, is, my right leg(I am righty). And, good ole goofy Lipton Commercial guy, Vic Braden, said, "You have to decelerate this mass, and accellerate that mass".. Or something like that. But, that still sits in my head. With the left shoulder has high as it can be while my right is as low as it can be, I push has hard as i can off my RIGHT foot, then hip, then drive my left shoulder down as the power in my right already built helps power my right shoulder UP. This is called the "biomechanical chain", and takes years to develop properly. Now, the arm?.. it is as loose as i can get it!.. As, basically, all the power i have developed is actually from my asterick, and my core(which all sports is key). It does help to actually get the ball in that i am 6 foot 2 and weigh 190 plus pounds. Yet, all the mass i have, plus my 13 ounce k90(used to be tour 90), gets transferred into that ball. Another trick is i only grip the racquet with thumb and ring finger and build up the bottom of my grip to allow maximum loose swingspeed without messing up my paint job on the ground. Only the PATH that i need my arm to travel changes and possibly my grip depending on what serve I am hitting. However, the basics of power don't change, and, everything that gets timed from the "trophy position" is ingrained, and, yes, takes much time to master exploding or 'accelerating that mass' into the ball. Yet, i hope that has cleared up the major differences. You can toss the ball 8 feet in front of you, and if you armswing, you will get only soo much mph. Remember, if i stood 8 feet behind the baseline, i could still power a ball, and would be going the same speed as my normal spot at the baseline. The contact point is only determined by ones height and the distance back they are from the net. Oh, and just one more fine point. The "hip forward", meaning the left hip, is only to balance the body and get ur asterick under you and allow the left shoulder highest and right shoulder lowest, and the right leg naturally bends and coil loads. Hope this helps, and if it does, hope you don't live near me, haha.