This new bag rider design, with the included flush cup, was an idea from Dave Preston, winner of the 2015 PRS Series. The bag rider attaches to your MPA BA Chassis, Comp Chassis or BA Lite Chassis that has a threaded three hole pattern in the bottom of the buttstock body. Its general purpose is to allow a consistent management of recoil when using a rear bag.
The purpose the flush cup, as conceived by Dave, is used when shooting certain positional stances when a sling is used to provide support for the rifle. Have the rear connection point on the bottom of the rifle pulls the rifle downward, as compared to side positioned flush cups which can trend the rifle laterally.