Williams flippers don't have the snap that Stern flippers do? *scratches head* Um, sorry, but no. Just turn the coil strength up on WOZ if you want powerful flippers. Of course you may break some plastics doing that.

A number of Sterns do not have a Hold winding in their flipper coil. They're using electronics to reduce the power to the coil when it reaches the end of stroke while a B/W switches over to the hold winding. It often makes a bit of a buzz when holding a Stern flipper up. I prefer the W/B design because you can adjust it for maximum stroke, tweaking the EOS switch out to almost the every end gives you quite a strong flip.

I think you're right on it being PWM. They're using a power triac to provide lower voltage to a single winding coil.