At risk of getting further off-topic, BYOND does nothing special in this regard, it does just enter as



Basically it's a host thing, where their HTTP server checks the origin domain associated with the request (sent by your browser) to check it matches one of their approved ones. Pretty standard hot-linking protection that you see over quite a few places, particularly on sites that are trying to run on the cheap, or have had prior issues with hotlinking, and are trying to use as little bandwidth as humanly possible.