[Overdriven]

They are supposed to allow less rebound damping (more rebound dampening = slower shock extension from the compressed position) so the suspension can return to ride height quicker after being compressed. So you can run the thicker shock oil (for more compression dampening) for bigger jumps and stiffer handling, but still have less rebound dampening like a lighter shock oil would. It basically allows you to run 100wt for compression, and say 60wt for rebound at the same time. I've never run them, but it sounds like a useful tuning too.