If your guitar has a single-action truss rod, then you’ll only have control over the amount of backward bow, with any forward movement either being built into the design of the neck or occurring from the pull of the strings. Many contemporary guitars use a double-action truss rod, which allows adjustment in both directions. Cranking clockwise will yield less relief, while loosening the truss rod counterclockwise will result in more relief.

Excerpted from Acoustic Guitar Care and Maintenance