The calling actor needs to have a valid target which corresponds to the bridge; this is normally handled by A_BridgeInit.
This function moves the calling actor to make it revolve around its target. The default speed corresponds to 0.84375° per function call (it should be called every tic for proper behavior), and the default distance is 15 map units. Custom bridges allow to override these values; though this is a property of the calling actor's target bridge rather than of the calling actor itself.
This codepointer is restricted to BridgeBall and derived classes.