[PaulM]

The 'emergency' is failure of major video systems.
It's a simple cut bus between sources which bypasses anything in the mixer which is complicated and has gone wrong.

A normal feature at the time, although designs varied, but it was often just a bunch of relays (or simple switches) hard switching the incoming video which may go right back to the sources (cameras etc.) taking out another level of possible trouble in distribution amplifiers and sub-systems.

Nothing complicated or sinister - just a fix in case of electronic and/or wiring failure to 'keep the show on the road'.

The one in the picture is under a lift-up flap to prevent accidental operation.

Not really needed today and difficult to do in a software controlled environment, but I think it's usually still an option (of sorts).

Best regards,

Paul M