It does exactly the same thing, just one blocks fluid inlet to the brakes (in the rear line) and the other blocks fluid in (when in the front line) Right?
Which also means the orientation of the lock needs to be reversed because most of them allow fluid into the brakes but not back out. (meaning pressing the pedal with the lock engaged allows more fluid to the brakes but not back out) until the lock is released.
Since this is the case, putting it in the rear brakes line would require that it be installed with the flow direction reversed. (so it wouldn't allow fluid into the rears while holding the fronts on)