vacuum advance distributor

A distributor that works by adjusting its speed based on the amount of vacuum coming from the intake manifold. Stock Type I Volkswagens had them installed. They are generally accepted to be inferior to the centrifugal advance distributor.

On a Type I Volkswagen you can check if it is working by taking off the distributor cap and pulling the cable off the intake manifold and sucking on it. If the gap between the contacts moves, then it's working. (I shouldn't have to tell you to turn the engine off before you do this).

The distributor speed is not affected by the amount of vacuum coming from the manifold. Its relative phase in the compression/fire cycle is affected.

The consequence is that the vacuum in the manifold rotates the rotor a relative amount via a mechanical linkage, and changes where in the cycle the spark ignites the petrol.

This has the effect of increasing the amount of torque and power that the engine generates.

Timing advance and retard is a slightly complex subject which needs to be understood if you wish to tune your own car.