I'd assume it just syncs the place. Otherwise, there would be a whole legal component to this that would likely result in a swift shutdown.

Actually, there's another product that does that and somehow gets away with it. They actually open netflix on a virtual machine and use a ton of hacks to make it look like its you controlling it. The performance is horrible though because there is two hops.

https://rabb.it/

Good question. It uses websockets to send/receive sync commands to each individual browser, each of which has its own instance of netflix. So theoretically every user still needs to have their own Netflix account.