A good explanation of how the ASA shock works (from their Facebook page
https://www.facebook.com/pages/ASA-Suspension-AB/137753293083108) I did a Quick translation with Google translate.
ASA system's principle:
Nitrogen spring protected by rubber membrane so it can not be mixed with oil. Pressure can be adjusted so you always have the correct spring without changing parts. The nitrogen spring never fades, it weighs nothing, but gives the absolute best performance in comfort and isolation from the terrain .
Damping in the return phase only, the oil flowing laminar between piston and cylinder inner mantle controlled by piston speed through the oil that automatically sets the column in the "moderate" position - ASA patent - so it Always has the correct damping force. No set screws, clicks, controls are needed.
With laminar flow the oil is not cut off or forced in the same way as the competitors.
Thanks to the high internal pressure, no water or other foreign objects get into the shock absorber.
Translation to the pic. Gasbehållare = nitrogen container, Olja = Oil, Gummimembran = Rubber Membrane, Stålkona = Steel cone, Plastkona = Plastic cone.