I was also confused by this at first but after looking at the explanation closely I caught it. The plunger itself has grooves, and when the pen is closed, the cam's grooves and plunger's grooves aren't aligned. When pushed open, the cam is unconstrained and the pressure from the spring below causes the cam to slide into the plunger's grooves. That's what causes the clockwise rotation of the cam.

Really cool site!