In order to obtain the mechanism for the infrequent phenyl ring it-flips in glassy polycarbonate, a generalized Langevin dynamics simulation was performed on a reduced model consisting of a flipping ring and its keeper ring. The frequency of pi-flips and activation energy for pi-flips obtained from the simulation are in agreement with experiment. A phenyl ring pi-flip occurs when there is an increase in the separation distance between the ring and its nearest neighbor ring on another chain, accompanied by, and in synchrony with, an increase in its rotational kinetic energy.