If you borrow bitUSD and the value of BTS drops you can buy back more BTS with the same USD - this is called a short position.

If, as you believe, the value of BTS increases you will get called and be forced to buy back the original BTS with more bitUSD.

Borrowing on the DEX is an incredibly powerful tool, however, you have to understand what you're getting into.

Just borrow $10 bitUSD and see what happens. Set your margin at 2.00 and see if you get called on the next leg up for BTS. Very cheap but very valuable lesson. Cheers!