There have been many different variations in truss rod design and technology since truss rods were invented. Two of the different kinds of truss are one-way or single-action truss rods and two-way or dual-action truss rods. There are two main types of one-way and two-way truss rods: compression and bending.
One-way truss rods are used to adjust the neck in a single direction: backward. One-way truss rods can be designed in any number of ways. I will explain a few of them. One design of a one-way compression rod inserts the truss rod into a curved or straight channel. One end of the truss rod is fitted with a hook or a welded nut and anchored in the neck. The other end of the truss rod is threaded and fitted with a nut and a washer. When the one-way truss rod is tightened, the distance between the truss rod nut and the other end of the truss rod is shortened. This shortening causes the truss rod to straighten out, if in a curved channel, and press against the neck. In a straight channel, the shortening simply compresses the back of the neck where the truss rod is inserted. The neck is forced to comply with the pressure of the truss rod and bends in an upward motion decreasing relief. Loosening a single action truss will not "bend" the neck in the other direction; it simply relieves the pressure from the truss rod and allows the string tension to bend the neck in the opposite direction causing relief. It is impossible to adjust the neck passed the "neutral state" of the truss rod since the one-way truss rod does not adjust in both directions.
A two-way truss rod, on the other hand, is used to adjust the neck in both directions. There are many different ways two-way truss rods are made, but the basic principle is the same. Two-way truss rods are placed in a straight channel and can either be made of one or two rods. A basic compression two-way truss rod is threaded on both ends of the rod in opposite directions: one end has a left thread and the other had a right thread. Both ends are fitted with nuts; the nuts are anchored in slots in the neck. When the rod is tightened, the entire truss rod rotates. This rotation causes the nuts to be threaded closer together causing the truss rod to bow forward eliminating relief. When the rod is loosened, the nuts are threaded away from each other causing the truss rod to bow backward creating relief. Another design of a two-way truss rod is the bending design that consists of two rods attached together at the ends. One of the rods is fixed and the other is adjustable in length. When the truss rod is tightened, the fixed rod will bow forward toward the fretboard. When the truss rod is loosened, the adjustable rod will bow backward away from the fretboard.
These are just a few of the designs of one-way and two-way truss rods to explain what the difference is between them are. For more detailed information about the different types of truss rods and how truss rods work, please visit the truss rod page.