Cutting out all the details, when you short a stock, this is essentially what happens:
1) Today, you sell stock you don’t have, at today’s price.
2) Later, you have to buy that same amount of shares, so you have something to transfer to the person who bought from you.
3) If the price went down in the meantime, hooray, you just made a profit.
4) If the price went up, boo, you just lost money.
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