After a certain amount of time, (min. 6 months in some states) L platers can obtain their P plates which enables the driver to drive un-accompanied.
However, the act of flippin' platers sees an L plater swapping his plates around (since they are double sided - one side has an L and the other has a P) to make it seem as if they are a P plater. Illegal but otherwise very useful until you get pulled over.
Guy 2: Flippin' plates!