>Prevents any move that targets the user from taking effect for the turn it is used in. Does not prevent passive damage such as sandstorm or poison. This attack has a 1/X chance of being successful, where X starts at 1 and doubles each time this move is successfully used. X resets to 1 if this attack fails or if the user's last used move is not Detect, Endure, Protect, Quick Guard, or Wide Guard.
Smogon.
Basically long story shot, your chance will decrease consecutively if you alternate between Protect and Detect in the same way as if you just spammed Protect
