There is a flat pressure bar inside the pen. One long edge sits in a shallow internal groove. The groove stabilizes one edge of the bar so the bar will pivot along that axis. A wire attached to the bar extends through the end of the pen and has a bead on it. It is pulled back to unlock it. Rotate the external handle 90 degrees, and it pivots the internal bar to compress the sac. The motion is a bit like that of a lever-type door handle (or flushing a toilet, if you prefer).