Manifold vacuum is very easy to tap and utilize: Simply drill a hole in the intake manifold and stick a hose in it. This vacuum can then be used to power accessories (headlight doors and heater controls), or it can be used as a signal source based on how hard the engine is working. One such signal source application is distributor vacuum advance: When the engine is working lightly (high vacuum produced), the ignition is advanced, and as the engine loses vacuum due to the throttle being mashed to the floor, vacuum is lost and ignition is retarded. Pretty simple. But what if you want to “switch” the vacuum on and off based on whether the engine is just idling or in cruise mode? Enter the term “Ported or Timed Vacuum .” Off at Idle, on with throttle open.