compress
lol jk but its used with threadless frok becuase they dont not have the big nut to tighten them onto the headset, threadless fokrs have a cap on the bottom near the wheel which the head of the long compression bolt sits on, the bolt goes all the way thoguht the fork with about an inch hanging out, then the bar are placed over the fork, which have a starnut in them (hope you know what a starnut is) the bolt threads into the starnut pulling the bars down onto the head set to *compress* every thing together and reduce play in the headset, hope this clears thing up