The spring half of a quarter-turn panel fastener system. These fasteners hold bonnets, boot lids, splitters, undertrays and access panels on race and rally cars, and the reason they are used instead of bolts is speed: a quarter turn releases the panel entirely, which matters when you need to get to an engine bay between sessions. The spring mounts on the structure behind the panel and the stud cams into it to lock. Springs take a hard life from vibration and repeated cycling, so they are a normal replacement item. Make sure the spring type, width and mounting method match your existing fasteners before ordering.