G7th Capo  
The wrap spring clutch consists of just three elements: a fixed hub, a moving hub, and a spring whose inside diameter is just slightly smaller than the outside diameter of the two hubs. When the spring is forced over the two hubs, attempting to rotate in the direction which opens the capo wraps it down tightly on the hubs, positively engaging them.
Rotation in the other direction, to tighten the capo onto the strings, is not restricted. To release the capo, the control tang is moved in the direction to unwind the spring, thus releasing the clutch.
The beauty of this type of clutch for this application is that it can be adjusted in tiny increments, allowing precise control over how  tightly the strings are pressed down.
Did you know... the G7th's clutch mechanism is a modification of a tried-and-tested design used in printers and copiers to provide a simple decoupling and overrun device.