Distributor: Description and Operation

Fig. 1 Breakerless distributor w/Hall transmitter:

The distributor, Fig. 1, contains a slotted rotor mounted on the distributor shaft and a Hall effect switch mounted on a plate inside of the distributor. The distributor has both vacuum and mechanical advance mechanisms.
The mechanical advance controls the advance in relation to engine speed, whereas the vacuum advance unit controls timing in relation to engine load.
On turbocharged engines the vacuum advance is a dual diaphragm unit. The second diaphragm is used to reduce pinging when the engine is under turbo assist.