The traditional-style steel truss rod is anchored in the wood on one side by a fixed metal-plate. On the other side it can be tensioned with a nut. To make it work perfectly a bowed installation is required. First a curved trench has to be routed, the trussrod gets inserted, and the rest of the slot has to be filled with a curved filler (stripe of wood) on top of it. After this the fretboard can be glued on. If the nut will be turned clockwisely, the rod will be tensioned and push against the wooden stripe from below, so the neck gets bowed to the back (backbow/convex). Some people say, that this traditional rod achieves best sound results, because it causes tension in the neck, and a neck can vibrate best, if it is tensioned...The installation of a traditional truss rod requires a certain minimum thickness of the neck. Extremely thin necks cannot be produced with this system.
The double-truss rod works as follows: Two rods, which are connected on one end, and one of them gets tensioned by nut against the other. For this truss rod only a flat trench is required, which will be directly covered by the glued-on-fretboard. By turning the nut, the bottom rod compresses the top rod and the neck will be backbowed (convex). If the rod has no tension, in theorie it can be taken out of the slot. Due to the straight trench, it is possible to create very thin necks with it.
Dual-action truss rods are quite similar to double-truss rods, but both rods can be tensioned in both directions, so it ispossible to achieve upbow (concave) and backbow (convex).