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The 75 ohm switch on a TBC or really any broadcast equipment generally means that it is the last device on the input chain and requires 75 ohm termination to pull the voltage levels down to SMPTE levels (roughly 1V peak to peak). If you were to hook a composite video signal up to a regular oscilloscope without a terminator, you'll get a peak to peak voltage of around or a little over 2V, whereas composite is meant to peak at 1V if appropriately terminated with a 75 ohm resistor which you an do with a BNC Tee and 75 ohm terminator. Devices like TVs and VCRs internally will terminate with 75 ohms automatically. Some devices can "auto terminate" without a switch. S-Video technically needs termination as well, but S-Video loopthroughs are rather rare to come by as it wasn't used as much in the broadcast world - composite and component was far more common.
Flipping the switch, you should see that the picture goes from being blown out (very light/bright) without it and normal looking with it on if it's associated with the input you are using and you aren't looping out to another device. If you are using S-Video in and the 75 ohm switch only affects composite, then you'll notice no change at all as it is not associated with the input you are using.
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