Not sure what difference it does in practice though. Perhaps not that much really. S1 as XR3i has restricting valves for the rear brakes in the engine bay, but they operate based on decceleration. The more a car decellerates, the more the pressure is restricted.

S2 has restricting valves bolted next to the rear suspension arms and these restrict the pressure when the arms are moving down . This of course happens when the rear end lifts up under braking and gets light.

So they more or less do the same thing, but are based in a completely different principle.