Clamp adapter embraces a lens body and holds it by friction force generated by clamping screws.

Such adapter is used when a lens has no any other way to attach it. Clamps are no 100% reliable usually, so some caution is necessary using them to avoid the lens damage. You should tight screws strong enough to hold the lens firmly, but not too strong since overtightening may result in damage to the lens body, jam its mechanism, or even break glass elements.