Its the emergency standby elevator control system. In the unlikely event of the elevator jamming, and the primary control separator failing. a signal is sent from the tertiary 'Q' pot simulator which effectively drives elevator control from the First Officer's control run, using the 'G' weight system.
The elevator is then automatically operated through the Flap Trim Compansator mechanism. It's a bit coarse, but better than no elevator control at all.
Darned clever these boys from Woodford!
Hope that helps - you won't see it on all aircraft, as it was superceded by the quadruple autopilot backup Channel 'C' installed on the RJs.
