That's a good question.

1. Have some BTS in your own wallet / Bitshares account
2. Go to any USD:something market. Preferrably something you like
3. Unlock your wallet
4. Click the "Borrow USD" button. A dialog will open.
5. Enter the amount of USD you wish to borrow. This should be no more than 1/2.5 of the value of BTS you hold
6. Adjust the "collateral" to 2.5. You will see that the amount of BTS held in collateral will change. It will turn red if you slide the collateral ratio too high. It means you don't have enough collateral to back the USD you are borrowing.
7. Confirm.

Congratulations, you have just created some USD out of thin air. You still have the same amount of BTS, but it is locked in to back the value of USD you created. It's insane, isn't it! Anyone can create USD without permission!

Now you can exchange that same USD for some more BTS or any other token on the network. If you buy more BTS, you are effectively shorting USD and long on BTS (you think BTS will rise in comparison to USD). However, beware that if BTS goes down compared to USD, you will need to lock in more collateral, otherwise you risk the system automatically "settling your position"; taking back your USD and returning you less BTS than you originally locked in. If you don't have enough BTS, you must buy more.

This is the way Market Pegged Assets are created in Bitshares. They can track the value of any non-digital or digital thing in the world, like oil, gold, wheat, Apple stock, or indexes. Gradually more and more of these will be created, and more BTS will be locked in, effectively reducing liquid supply, and thus increasing the price.