When you hook up to a trailer, the brakes are locked. When you add the air to the trailer (push in the red valve), that releases the brakes, which causes the trailer to move slightly. Then, when you turn the hand valve down, you re-engage the trailer brakes. So when you release the hand valve, it unlocks the trailer brakes again, usually bouncing the trailer slightly. If you think about it, the same basic thing happens in a car when you engage and un-engage the parking brake, just less so. Also, the same thing happens to the back box of a set, when you're hooking up, if you turn your red valve before the blue valve, which we're trained not to do. It's a little unnerving if you turn that red valve first and see that back box move up on you. But same idea; by adding air to the back box, you're releasing the brakes.
