It's a vacuum gauge. Partial throttle produces high manifold vacuum. Open throttle produces low manifold vacuum. Actual response depends on the car, etc. My BMW is completely different from the MBZ's. The 300E will dip initially into the red, but then stabilize somewhere above that after vacuum is stabilized. Don't get wigged, those gauges don't give you any really useful quantitative info. Any car driven without mashing the throttle will give you better mpg than the car which lives under a "throttle masher".
1987 BMW 325