They slide when you fire.
It is not really adjustable in the mechanism. The more expensive ones also have adjustments, but the base system has nothing to do with adjusting how you shoulder the weapon.
When you fire the gun slides down the one plate instead of back against you.
Someone indicated this slows the recoil down reducing felt recoil rather than truly redirecting the recoil.
Only works for singles. Would never try it on anything else, but then again a BT99 isn't so hot for doubles anyways
I imagine you don't see many b/c the basic is $149 and the adjustable is $249.
As much as I liked it, I would have to be making a bit more money than I am now to buy another. But, hopefully, someday...