The same way you short stock. You borrow a bitcoin, promise to give it back, sell the bitcoin for its current price. At a later time you buy a new bitcoin and g

The same way you short stock. You borrow a bitcoin, promise to give it back, sell the bitcoin for its current price. At a later time you buy a new bitcoin and give it back. If the price dropped you turn a profit, if the price rose you'll have lost money.