XXGOMXX

Are they mechanical devices? and how do the fit?

nom

Can be... (I think there's a Dawes that can be rigged as such?) but normally the 'off the shelf' product is a small electroic widget in a box that needs to tap into a couple of cables in the wiring loom next to the ECU.
Pretty easy to fit if you're handy with a soldering iron (you can attach it with vampire clips, but may get strange beahviour/CELs) - the hardest part is probably identifying the correct cable to tap!