http://www.skimetrix.com/
I bought a pair of these for hardboots, but also found it quite useful for soft carving. Two options to use:
1. Put it around the top of the cuff of the boot, just like you would on a hardboot to get a super snug fit and medium response.
2. Thread it thru the back of the highback of the binding, put your foot in as normal, connect the normal straps, then tighten down the Booster strap as a 'third strap'. This keeps the boot against the highback for a hard response.
I tried this because I was having trouble getting enough response out of my Burton Mission bindings and Salomon Malamutes on my Burton Frontier 175. It works wonders, as close to the hardboot feel on a softsetup as I have experienced. If you want to retain some of the surfy feel off piste, just add the Booster on the front binding so you have some play in the back. Best of both worlds and you can always remove it for the really soft days.