Timothy Walters Kleinert
After you, err... "pulse outward" with your center, do you actively "retract"? Back in my karate days---and earlier in this thread---it was advised that one should pull the hand or foot backwards after the strike. (There's a certain tactical advantage to this, but it's hard for me to see how it contributes to power generation.)
To build on what Mike said, if you have two balls of equal mass, ball one is stationary, the ball two rolls into ball one with a given "speed", then under ideal conditions, ball two will stop, and ball one will roll away with the speed ball two had. Basically, the speed lost by ball two, equals the speed gained by ball one. So if we add some spin, and we make ball two bounce back after the collision, then the change in speed from positive x to negative y, is larger, hence ball one, will now have a speed equal to x+y.
Hence the snap back increases the change in momentum. The force of the strike is then given by the change in momentum, divided by the time the strike occurs over. Hence a quick snap back, also increases the force.
This is the opposite principle behind an air bag in a car, which increase the time, decreasing the force.