This is used used by the ECU to measures differences between atmospheric pressure, intake manifold pressure and also low pressure (vacuum). Pressure within the intake manifold changes with position of throttle. At WOT (wide open throttle )manifold and atmospheric pressure is about same which give out a high voltage reading. When KOEON(key on engine on) the pressure inside the manifold lowers creating vacuum ( a low volt reading). Oxygen content and air pressure also changes with altitude ( high altitude low pressure low altitude high pressure) which then ECU needs to compensate by making changes to fuel flow either by increasing or decreasing the flow rate entering the engine. This is archived by having a MAP/MAF,BARO and BMAP. The ECU reads the MAP to determine load on the engine and speed which then adjusts the pulse width of the injectors which alters air/ fuel mix and advances spark timing. It also detect deceleration with the drop of voltage which is seen by ECU and then fuel will either be stopped or majorly reduced to improve emissions. A change in the resistance causes change in signal voltage of intake manifold pressure.With the vehicle at sea level, if the throttle is open the manifold absolute pressure will increase from 15" Hg to near 30" Hg (mercury~100 kPa), about equal to the local barometer.The higher absolute pressure in the intake manifold increases the air's density, and means more fuel can be burned resulting in higher output.
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