http://www.motorsport-developments.c...whatdoiget.htm

Some info there.

As Rick says, with Cossies and RST's the original chip is replaced (RST require soldering, Cossies are on a removable socket). The relevant maps (fuel, boost, ignition etc.) are modifed, burnt to a new chip and replaced in the ECU, then the ECU operates as 'normal' but with the new maps for increased power etc.. so Cossies/RST's the chip is a replacement for the original.

On something like a Pectel/SECS/Autronics ECU, which are more modern, you normally get mapping software and the ability to plug into a laptop to directly change the relevant data.

A common method of 'chipping' (generic term, as per Chip-3dr) a Japanese car in the past, was to fit some kind of 'piggy back' device which intercepts the std ECU signals and modifes them in some way. This was largely because tuners didn't have the ability to read/modify the std ECU's.