Formula has foam spacers, so they compress with rising air pressure in the chamber and give some more linearity - with added plastic volume spacers, you ramp up the spring very quickly, this takes some of that ramp out. As for the decoupler, they have a floating-ish piston, which means they lessen the piston bind when the fork flexes. What the buttercups do is different, they take away the chatter that isn't covered by the spring in the fork (due to stiction and the like). So the same as a rubber top mount in a car (with flexible rubber bushings...