What does it mean to have fast or autonomous switching "enabled" and "disabled" on the same interface?

Autonomous and fast switching are the words used interchangeably.

How packets are switched will depend on the for which switching mechanism outgoing interface is configured.

If you enable fast or autonomous switching on an interface, packets coming from any other interface on the router are fast-switched  to that interface. If you enable it on same-interface , packets whose source and destination address are the same are fast switched.

Here same interface simply means packets entering and existing the same interface.