From the Bos website:
An impact on the wheel will be translated on the damper by a displacement of the rod, on a certain lenght, and more precisely at a certain speed. Then, the behaviour of the whole suspension is handled by a control of the speed of the rod, which induces a more or less quick deplacement of the oil through the piston’s ports. That’s the reason why, the adjustments are differentiated as High and Low speed. All in all, the low speed compression runs the châssis handling (control of roll, diving), and the high speed controls the impacts inducing a fast or sudden entry of the rod. Therefore, it’s easy to understand that high speed compresion doesn’t mean necesserally that the damper will take a lot of travel.