Before i get shot down by tech guys this is in laymans terms and not strickly acurate.
TPMS works by reading the signal from the anti-lock brake sensors.
The sensors are next to something that looks a little like a cog on a bike wheel/chain (think of a bicycle wheel with a cog on without the chain)
As the cog turns with the cars wheel the ABS sensor sees a gap or a tooth on the cog and it generates a voltage signal that changes depending if it sees a gap or a tooth. This changing voltage tell the computer how fast each wheel is turning (as each wheel has a cog and a sensor).
What the TPMS does is see one wheel is constantly measuring different to the others because its circumference is different due to the tyre been flat (one tyre is flat so the wheel is a smaller circle so middle of circle were the cog is, is different speed to the other 3 wheels) so the speed of the cog has changed compared to the other 3 on the other wheels. This change is more or less what triggers the warning light from TPMS.
Every GTI has the ABS sensors, the TPMS warning light and the software in the computer just not the reset button and wiring for the reset button. What Am1W has done is turn on the software.
