On the left is the stock reverse boost valve and plug, and the BD reverse boost valve and plug are on the right. The BD valve has been bored out in the center to accept a check valve, and the plug has a smaller orifice to increase reverse line pressure from the factory setting of 260-300 psi to 360-400 psi. This mod is beneficial for people with trailers, heavy campers, and off-roaders who may have to back up on steep hills-the extra line pressure prevents slippage. The check ball, meanwhile, is what allows the option of Second Gear converter lockup when used with a BD Tow Loc. This is convenient for heavy trailers that may descend steep grades.