They're floating pucks. They are held in place laterally by the holes they sit in, and the 2 pressure plates hold them in their place. When the clutch is disengaged, there's only a millimeter or two that the pucks can possibly wiggle around in, and when the plates engage, they press together, grabbing the pucks. Since they cannot move from within their holes, the entire assembly is forced to rotate as one.
An example of another thing that uses this design is the old-school Traxxas RC car slipper clutch.
The red circles are the clutch pucks, and they can easily be pushed through the holes in the gear. However, the plates to the left and right of the gear sit on both sides of the shaft and press against the pucks, holding them in place. By varying the tension, you can adjust how much the clutch slips.