The Bi-Flex Truss Rod (used on most American and American Deluxe Series instruments) was designed by Fender in the early 1980's. Unlike most truss rods, which allow only convex neck adjustments (back bow), the Bi-Flex allows you to adjust for either a concave or convex bow. If you tighten the truss rod nut it bows the neck backwards. As you loosen the nut you will find the neck's neutral position (the truss rod is not in use). If you continue to loosen the nut you will feel a renewed tightening as the rod pushes against the walnut dowel causing the neck to bow forward.