I'm sure someone can give a better explanation but....as it is an open system (i think) when you push the lever when there is no rotor in beteween the pistons, the pads will move out and stay there.All you need to do is take the rotor out, and use a small scredriver the seperate the pads/pistons apart, so they are back to normal.It's best to take your pads out, so when seperating the pads, you don't damage them Hope i've helped