In the best case, you never pay it back. In a good case, you make a small profit, which you get to keep -- as well as whatever profits you made from your other investment. Your returns are literally infinite, because you started with nothing.

They won't actually let you do it with zero money, but it is a way for somebody with a very tiny stake to multiply it quickly.

The flip side of the "infinite" return is the "infinite" loss. And that's way too rich for my blood. But there are those who like taking risks. And unlike other get-rich-quick schemes, it's not a pyramid scheme or a scam. It's just a bet that you're smarter than the market. (Though if you don't realize why that can be a sucker bet, then it is surely a sucker bet.)