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A vacuum hose connects the intake manifold to various parts of the engine that rely on air pressure to function.

When you let off the gas pedal, the throttle plate closes, which means that the intake manifold is closed off from the outside world. As the engine is still turning and trying to take in air to burn with fuel, it creates a low pressure inside the intake (what people call a vacuum). If you ask a scientist, there's really no such thing as a vacuum, just a low pressure relative to the outside world.

Many systems have a vacuum powered break booster. Many times you apply the brake after letting off the gas so car manufacturers thought it would be a good idea to use that low pressure to give you a little help when applying the brakes. This was especially obvious in my last car ('92 Saturn). As it would downshift as I was braking, I could feel the brake pedal response change as the pressure varied in the intake.

In our 1.8T engines, the Diverter Valve needs to open when you let off the gas to the turbo can spin freely and not damage itself. It knows to do this because, again, there is a low pressure in the intake. So a "vacuum" hose is run from it to the DV to tell it when to open.

Low intake pressure also tells the engine something about how it is idling, but I haven't learned exactly how yet. Many times, if you have a "vacuum" leak (bad hose) it will cause your idle to hunt around as the ECU tries to figure out exactly WTF is going on since that sensor and the O2 sensor are feeding it two conflicting pieces of info.

There are other uses as well, but I can't remember them. Hope this helps.
 
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