The DAV slot just connects a card to the factory A/V in and out ports of the host machine (bypassing the original capture hardware).

I have one of those AVID PCI cards in a 6500 but can't use it untill I trade out the A/V card for one with the correct DAV connector on it (mine is too small).

The DAV on the 840av is a special slot inline with one of the NUBUS slots, the Supermac SPigot AV and AV Pro are the only cards I recall that will use it (I have the Pro AV in my 840av).