The Townshend Audio Seismic Pod is a spring inside a rubber sleeve, with a tiny hole that allows air to escape when the spring is compressed (somewhat reminiscent of a bellows). The spring provides isolation, the rubber sleeve and compressed air damping.
In one of his You Tube videos, Max Townshend demonstrates the difference between the bare spring from a Pod vs. the complete Pod. Easy to find on You Tube, just do a search for the company name.