Basically all it does is shut off the flow of brake fluid to the rear brakes. The adjustment is critical to prevent the back brakes from locking up! If the car has been lowered then the amount or rear end lift will be greater to get the valve to operate, by this time the rear wheels will probably be off the ground and the rear wheels will be locked!

In the above pic the adjustment arm on the left hand side would be rotated anti clockwise (viewed from the rear) when the vehicle had been lowered. The right hand side adjustment arm would be rotated clockwise.