The thing with the Toaster is that you need to syncronize up to 4 external video sources with each other and the computer. A Genlock only has to sync the computer to one external source. That means the horizontal and vertcal scan rates must match, as well as the color burst clock on all four sources. Basically, the TBCs have to digitize the video and clock it out in sync. The Genlock just derives that color clock and sync pulses from the external source and uses them to lock the amiga to them.
