To help energise the airflow over the outboard section of the wing to ensure the tip didn't stall. The a/c had extended wing tips as they are -500 airframes and had a problem at high speed where a jet upset situation could end up with the a/c 'winding up' to excessive G. There were several systems in the a/c to help prevent the problem which was initially triggered by the active ailerons simultaneously pitching up to relieve G load caused by a gust. These systems were called ACS (for active control system) MDLC (manouvering direct lift control) and RSB (recovery speed brake).