Bags are constructed from reinforced heavy duty PVC and do not rapidly deflate if punctured. Draper supply two protection mats to protect the bag. Connect hose to the car exhaust to inflate; connection coupling is a valve that stops the bag deflating. You turn it slightly and the bag slowly deflates lowering the van or car to the ground.
At odd times when we've needed to use it on the caravan it's just placed under the chassis and an axle. We used it to lift the van when it sank into soft ground on a CL and you could not get a normal jack under it, or use safely.
MS search found this with photo's and instruction sheet download on the page
http://www.tooled-up.com/Product.asp?PID=18521
Easylift are the number one airbag jack but prices are probably a little steep for caravanning use.
http://www.air-jack.co.uk/
Amazon sells Draper airbag jacks. Apart from placing the bag in position and slipping the coupling cone on to your cars exhaust you need no physical effort to use the jack so they are ideal for those with limited physical ability.