The number one source of power on a serve is pronation - the turning outward of the hand and arm on impact. You can have pretty lousy fundamentals, but if you get this right you can still crack a serve. This is why pros can warm-up at about 40% and still hit extremely hard serves. This is why you can hit an overhead that hits the ground and then jumps over the opposite fence. The torquing motion is like a mini atom-bomb in terms of power generation.
In Sharpy's serve, he isn't getting any pronation at all: