At 2:34 in
this youtube video, there is a closeup of a cuff with a see through side. It never was mentioned in the video (on how to get
out of cuffs), but there is a straight horizontal part on the bottom of the cuff (I'll call it the
stop bar). That part has two knotches and the spring is sitting in the
left slot. When the key is inserted and turned to the right, the pawl will go down and the ratchet can back out and the cuff is opened. When it is opened (or just being tightened) the pawl will slip into an indentation of the stop bar. Pushing the stop switch in (and moving the stop bar), the spring will now rest in the
right slot and the bottom of the pawl will be sitting on top of the stop bar just to the right of the indentation, preventing it from moving. At this point the end of the stop bar would be under about the 7:00 spot in the keyhole. Turning the key to the left between 7:00 and 5:30 ish would unlock the double lock and allow the key to be rotated back to the right to release it (2:00 - 3:30 or so).