3. 1 month passes. The stock goes up.
You HAVE to buy the same amount of stock, but for $150.
You HAVE to return the stock to the person you borrowed it from.
You lost $50 per stock.
You HAVE to buy the same amount of stock, but for $150.
You HAVE to return the stock to the person you borrowed it from.
You lost $50 per stock.
What does happen if the stock goes up is that your brokerage company will ask you to put up the delta. In other words, if it goes from $100 to $110 you'll be required to deposit the equivalent of $10, the delta, times the number of shares you shorted. If you shorted 1,000 shares you'll have to deposit $10,000 for every $10 of upwards movement in the stock price. If you have long (traditional stock buying) positions in your account your broker might actually sell those automatically to cover this delta.
The other important point is that this is a loan. Which means you will pay interest on the funds, in this hypothetical $100,000. The interest charged can vary. If, for the sake of an example, we assume 5% simple annual this means $5,000 per year or just over $400 per month.
I used to day trade (about 20 years ago) and would use shorting multiple times per day. I am not sure I would consider shorting for long term (> 1 day) positions. As many have said, the potential for loss is great.