Turn the key to the "on" position without starting the engine. Notice that the boost guage will read about 1 bar (less if the car is at higher altitude, should read exactly 1 bar at sea level).
When the engine is running there is a suction through the intake which makes the pressure lower than atmospheric, the turbo isn't making boost at idle. Notice that the boost guage reads somewhere around 0.4 bar with the engine idling.
0.0 bar is a complete vacuum, like outer space.
0.4 bar is a _relative_ vacuum since the pressure is lower than the outside air pressure.
At full throttle, a non-turbo car will have an intake pressure roughly equal to atmospheric (1.0 bar). To get the intake pressure above 1.0 bar, a supercharger of some sort is necessary (a turbo is actually a sub-species of supercharger, but that's getting really technical).
Most boost guages show 1.0 bar as "0.0" because it's the normal air pressure and the car can achieve 1.0 bar intake pressure even without a turbo or supercharger. "Boost" is defined as manifold pressure above and beyond atmospheric pressure. So 11.76 PSI (0.8 bar) of "boost" pressure is the same as 1.8 bar of "absolute" pressure.
The guage in the 951 is really a manifold absolute pressure guage and not a "boost" guage because of the way it is scaled.
Aaron