It doesn't. They're almost always open collector/open drain signals, with the pull-up on the host (PC) side. No power needed, if you've got mechanical switches doing the commutating!
(Of course those aren't so reliable over the long term, so transistors are usually used. But swinging a base or gate around is all that's needed, and that is easy to do with almost any available power source. Like the main power for the fan!)
(Of course those aren't so reliable over the long term, so transistors are usually used. But swinging a base or gate around is all that's needed, and that is easy to do with almost any available power source. Like the main power for the fan!)