Let me get this straight.
So the rear brake has delayed engagment due to the clearance while the load is being transferred and the front brake is working as normal. (So you get maximum front braking, and sacrifice the rear to prevent lockup?)
Then, as you push harder the spring engages which takes force away from the rear brake, still the front engaging as normal. (There is still some reaction force holding the spring, so the rear should be getting some force, albeit not very much)
Then, as you push at the extreme, the spring coils bind and you get maximum rear braking force. (Is this at the turn in for a corner, after the car is going much slower?)
Thus, the wedge affects the clearance and the preload on the spring?
I agree with Dave as I would not want the 'push back' of unloading a conventional coil spring, elastomers are noted to provide a degree of damping, hopefully controlling the stored energy.
Looks like a nice mechanical solution to a proportioning valve, although I thought they worked by reducing front brake line pressure, not by reducing the rear, but F1 has so much load transfer that the rear is probably really easy to lockup.
I love to love Senna.