Derek: Like Dynamics and others said it fools the ECU into altering the fuelling by kidding the system that the air temperature is warmer or colder than it really is. Remember those tests I did a while back with the IAT and fuel pressure? If you remember I had to pull the 02 sensors to force the ECU to run open loop, otherwise the ECU just dragged the AF back again irrespective of the IAT reading. The device in the link is just a pot with a fancy ali **** on it.

Here's a section from the testing (sorry, not had chance to document evertyhing and post it on my site yet):