Touchdown is a very simplified form of a bypass. Instead of flowing through a tunable port, the ID of the shock body flares outward for the top 10% of travel so that oil can flow around the piston. With bypasses as seen on trucks, each bypass acts more like a position specific HSC. External bypasses are as seen above. On Fox internal bypass shocks they have an inner and outer shock body so that oil can be routed through the area between them. The oil flow on those shocks is controlled by a sort of finger shaped shim stack. Trucks...