We've already talked about what is truss rod; but do you know there's 2 main kinds of truss rods? Today, let's talk about single-action vs dual-action truss rods a little.
Single-Action Truss Rod
As the name suggests, single-action truss rods only function in one direction to balance the string tension and keep the neck straight. They are simple with a single piece of steel stick that is threaded on one end to receive an adjustment nut (which can be removed) and a washer, an anchor on the other end, needs to be installed in a curved channel. Unlike dual-action truss rods, this kind of rod can only bend the neck back. It cannot cause a forward bow.
Dual-Action Truss Rod
Double action rods, also known as 2-way rods, are typically made of a fixed flat bar and an adjustable round bar that is welded together with threads on both ends. To enable accurate adjustments, the adjustment nut is often welded to the end of the rod and can't be removed. They can not only counter string tension, but add to string tension effect on the neck relief. As you screw the nut, the round bar with bow backward to counter the spring attention, and as you loosen the nut, it will go back to the flat position.
Typically, a neck will be straight with no string tension and pulled upward under full string tension. In this case, a single-action truss rod is all that is required. But if you had a rigid neck or low-tension strings, the string tension might not pull enough. In that case, a double-action truss rod would be required to make the adjustment, by adding the required additional tension that the strings were not supplying.