Being totally immobile, I am prone to getting pressure sores known as bedsores. To avoid getting them, I would have to be turned often or be on a pressure-reducing materass or overlay. An overlay simply is placed on your standard materass and is held in place by a couple of straps.
The overlay has a small control box that hangs from the foot of the bed that contains all the control circuits, valves and compressor. The compressor runs very quietly and can be barely heard in the stilness of the night and is inaudible during the day. It works exactly like the EASE Wheelchair Cushion described elsewhere - inflation and deflation of various sections along it's length. My materass is labelled Auto Logic and is sold by ArjoHuntleigh (as far as I know).
This is my second unit - not because the first stopped working but because parts were becoming hard to find as it was now obsolete. Also, my previous one was an overlay (fitted over a standard materass) while the one I have now is a materass. I still use a thin foam materass to increase the overall height.
Another feature of the new materass is that it has a battery backup - in case of a blackout, it will keep working for up to six hours.