The rivet(s) you are using is like a ball headed nail in a tube (Barrel). When you start squeezing the handle, the nail is pulled into the tool, the ball end is pulled into the tube, widening the tube as it is drawn deeper. You keep pumping the handle until it pulls so hard that the nail end breaks off, leaving the ball end in the barrel, making the barrel snugged into the hole, which is now holding the two piece together it was slipped through.