Down under the breaker plate in your distributor are a pair of flyweights
restrained by a pair of springs and a pair of stops. As these flyweights go
faster they overcome the spring tension and are linked to the upper distributor
shaft which is concentric with the lower one but can turn some in it. The upper
shaft has the point cam and the distributor rotor. As the flyweights move
outward they cam the upper shaft forward (in the direction of rotation) so that
the points break sooner and also the rotor is still pointing at the right
place when the points break. If you take the breaker plate out of your
distributor you can see this riggin'.

Most Amazons don't have vacuum advance. Later Volvos have vacuum advance
and some even have vacuum retard. My guess is that most Corvettes have both
vacuum advance and centrifugal advance.

Vacuum advance is more to keep the engine from kicking back against the starter
than anything else. Vacuum retard is to enhance the idling emissions control.
Both are out of play at full throttle because when it is wide open there is
no vacuum.
--
George Downs, Bartlesville, Oklahoma, Central US