tension-clip

from The Century Dictionary.

• noun A device which, by a stress of hauling or pulling, is caused to grip a moving element, while a release of the tension causes the grip also to release: used in cable-railways or continuous wire-rope elevators. The usual principle is that of an oblique pull on the clip, which causes it to cant sidewise and bring a side pressure against the moving element sufficient to cause friction and prevent slipping when the effort comes on.