The air only pushes the clay with a very average flow (it doesn't need to be precise) to feed the auger that precisely (by changing the revolution speed) feeds the clay through the nozzle.
Here is the system that I bought:
http://www.vormvrij.nl/homepage-pictures/new-landing-index.jpg
Because cartridges are in plastic, they can't be used more than 6 bars (and only 3 bars when temperature is over 21C, which is often the case in Sydney!). That's the reason why I started using scuba tanks instead of those plastic cartridges to use them up to 8 bars, the limit of my air compressor. Now I would like to push the limit even more by trying with greater input pressure.
The extrusion screw is a better system I think, but more expensive as well (and I have no idea how I could design that. So far, I just improve a system. Going with an extrusion screw would mean to re-design it completely...)