I must admit when doing the functional check for the RPS I never really understood what it was for until now !! Incidently we did not check the HUD symbology during the functional.
JF has it all here in his reprinted lecture...
John Farley's Lecture
During the early 1970's a package was developed to help the pilot deal with the issue of sideslip during mid transition where intake momentum drag makes the aircraft directionally unstable. An autostabiliser working through the yaw reaction controls was produced to reduce this instability. Additionally, an indication was provided in the head up display (HUD) of a safe limit of sideforce, allowing the pilot to limit any lateral out of trim that might otherwise result in a loss of lateral control. Finally, in case the pilot was distracted from noticing the HUD warning, rudder pedal shakers were fitted which operated as the HUD sideforce limit was reached. The system shook the pedal that the pilot needed to use to reduce the sideforce, not only reminding him to use his feet, but also indicating which foot.