Regen is simply taking the fluid from the rod side of the cylinder as it extends and directing it into the piston side of the cylinder along with pressurized fluid. This allows a loaded bucket to aid the piston side flow and prevent cavitation. Since regen happens right in the joystick valve, it is effectively tying the piston and rod sides together with a check-valve so pressurized fluid cannot flow backwards into the rod side. If the pressure on the piston side is very low as it would be in a cavitation situation, the rod side easily flows into the circuit along with fluid from the pump. As soon as any backpressure is felt, all regen flow stops because of the checkvalve action.