Two way action can correct either a backbow or upbow on a neck.  Twin steel rods and brass threaded blocks allow for precise adjustments to get the perfect alignment on the neck. 4mm hex wrench included. We suggest creating a slot about .25 inches wide .5 inches deep in the neck. Make sure the slot is deep enough so that fretboard does not rest on the brass nuts. Also use a dab of E6000 or silicone caulk under each nut to prevent them from moving, do not glue the rods. The slot should be long enough to conceal the adjustment nut under the neck and/or nut.