The tension control was, and remains, the first of its kind: you simply squeeze the capo to attach it to your guitar neck. Removing is equally simple by squeezing the release lever. This allows extremely intuitive and infinite control over the pressure placed on your strings and heralded a new standard of tuning stability from a capo.
The mechanism is called a wrap-spring clutch; it allows the capo arms to rotate freely when closing the arms together, but if you try to rotate in the other direction, the spring tightens around the hubs so the capo cannot open. When you squeeze the release lever the spring coil is loosened, allowing movement in both directions so you can take the capo off the neck.