The mechanic

"The Shear Line": a plug turns only when every pin's joint sits on the shear line at once, and the key does it by a cancellation — the bitting terms cancel to one height. Scroll drives the key in; at the sixth joint the plug turns and the bolt withdraws.

The control case. The same cylinder cut two increments deep at one chamber — five joints on the line, one across it, so the plug never turns.