There's a display unit which sits on top of the dashboard, which reflects a display off the windscreen. For better visibility a diffuser is supplied which sticks on the windscreen. There are two levels of lumination, so that the display dims at night.
This display is the boost gauge. It reads -490 mmHg which is vacuum as the car is idling. I can't remember the exact conversion but that's close to 0.5 bar. When boosting the display changes to kg/cm3 which is very close to bar. The maximum reading is 1.6 which should be fine for most chipped Audis).
The display also has a analogue part which is the oval track around the digital display. As the reading increases, bars on this track appear and trail around the display just as the needle would on a mechanical gauge. There are also two warning LEDs, one amber and one red, which are fully programmable to light when a boost threshold is exceeded, and to flash when it has been exceeded for a few seconds.
In turbo timer mode the digital display simply counts down the minutes and seconds, whilst the analogue part shows the vacuum.
The whole unit is very well designed, consisting of 4 boxes which are interconnected with cables. The turbo timer box is hidden away under the dash. The display box sits on the dash. There is a control box which has buttons on which are actually rarely used once set-up is complete. So I mounted mine upside-down in the little compartment near my right knee (on the S4 that is). There still is room for a CD wallet too.
Finally there is a boost sensor which needs to be mounted as close to the air line as possible. Mine is mounted in the same compartment when the ECU sits, but on the passenger side. Thus there is only about a 20-30 cm run of vacuum pipe from the sensor thru a grommet to the T-splice in the air line which goes to the fuel regulator.
The control unit needs 12V from ignition, a signal from the lights (I recommend full lights, not side lights) to dim the display, and an earth wire. The 12V and earth can be obtained from screw-terminals near the fuse-box, and the lights signal from the back of the fuse-box (need to strip back some wire insulation and solder on the new wire).
The turbo timer is effectively a switch which temporarily overrides the ignition. So again there are two wires (a third is not used) which need to be soldered onto wires at the back of the fuse box. Finally there is the handbrake sensor wire which needs to be spliced to the handbrake sensor (or in my case earthed).
So fitting this unit can easily be done in a day, or less if you are experienced.
Note however that my turbo timer does NOT idle the engine. Instead I have modified Audi's own design as detailed in this post.
The Defi-HUD unit can be obtained from Nitosport in the UK.