if multiple "video slot master" cards are installed (such as the Video Toaster or a genlock), only one of them can be active at a time
a front panel selector lets choose which card is actually in control of the Amiga video slot
passive devices (such as the DCTV or a flicker-fixer) can remain active all the time
all four cards can be disabled giving the ability as though nothing is in the video slot of the Amiga - useful with higher frequency scanrates of the AGA chipset