Pretty close. People pay minnowbooster for votes.
Minnowbooster uses steempower delegated to it. Minnowbooster divides up the money it got paid for votes and distributes it to the people who lent steem.
It doesn't just upvote promising content. It upvotes based on being paid. But... since you can get downvoted, you can't (or shouldn't) pay to upvote trash or you could just be throwing your money away.