The Video Toaster had a simple setup in the days before digital - whatever you fed into Input 1 became the reference timing source and everything else got genlock'ed to it. This worked well enough for even TV broadcast needs, allowing public access and smaller TV stations to start using it.

Even more interesting, the developer manual:

https://discreetfx.com/documents/NewTekVideoToasterDeveloper...

I remember using this thing when I was a kid, trying to figure out how all the switching effects worked, so stumbling on this manual many years later was really satisfying...