Doesn't work half the time then!!! Number of times I've ended up sitting on the floor with the reins in my hands and the bridle on the floor.
If you do them up as loosely as I was taught (so you can get four fingers between it and the throat) it seems to have no effect at all as there is enough slack for the head piece to pull over the ears. At the RS I used to go to the numpty kids pulled the bridles off without undoing any straps!!