Analog TV studios used a master framing source for all their equipment so that the composite video frames were synchronized. Without that synchronization, switching video sources causes the monitor to resync to the new source and in the process the picture gets scrambled until it locks to the new sources framing pulses.
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https://en.wikipedia.org/wiki/Composite_video
Analog TV studios used a master framing source for all their equipment so that the composite video frames were synchronized. Without that synchronization, switching video sources causes the monitor to resync to the new source and in the process the picture gets scrambled until it locks to the new sources framing pulses.
NTSC signal explanation:
https://www.youtube.com/watch?v=X24zDXz8Xgc
Part2 Color Burst
https://www.youtube.com/watch?v=mHXDX0MXcyk
Here's someone who appears to have done it.
https://electronics.stackexchange.com/questions/116052/how-to-make-a-simple-video-switch