The dip in the bow is to allow the jailor to insert his thumb since the cuffs need a hard pull to open when unlocked.
Incorrect operation can damage the cuffs.
Rankin Leg Irons Rankin legirons were used during the Civil War period. This pushes in the spingloaded tube surrounding the threaded keybolt. The tube only moves in a short distance, so stop screwing the key in once you feel light resistance to rotation. This moves a wedge back enough that the cuffs will open with some slight resistace. Release the outward pull on the key, and firmly pull open cuffs by hand.