The chain fails to disengage from the bottom teeth of a front chain-ring ; instead the teeth snag the chain and carry it up and around the rear circumference of the ring, winding it back onto itself, and jamming it between the chain-rings and chain-stay.

Because it usually occurs unexpectedly during forceful pedalling, the chain can be permanently twisted, teeth can be damaged, and chain-stays of aluminium or carbon-fibre can also be damaged by the chain being wedged hard against them.