[rebootthesystem]

That's not quite right. You can hold a short position for as long as you want. There is no contract saying you have to close the transaction in 30 days. In that sense the otherwise excellent analogy provided isn't accurate (which isn't a problem, it's meant to be a simplification).

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.