can cover the short position. When a stock that you hold a short position goes up, you need more assets in your account as margin. If you hit the threshold for margin requirements, the brokerage will issue a margin call saying that you either need to exit the position or deposit more assets to your account. They give a deadline to do this. If you don’t satisfy the margin requirement before the deadline, they will close the position for you (regardless of any losses you will incur).