And what exactly do I profit from the short? I understand it is the difference in the value of the stock. So if my initial investment was $4000 (200 * $20) and I bought it at $3800 (200 * $19) I profit from the difference, which is $200. Do I also receive back the extra $2000 I gave the bank to perform the trade?
Either this is extremely poorly worded or you misunderstand the mechanics of a short position. When you open a short position, your are expecting that the stock will decline from here. In a short position you are borrowing shares you don't own and selling them. If the price goes down you get to buy the same shares back for less money and return them to the person you borrowed from. Your profit is the delta between the original sell price and the new lower buy price (less commissions and fees/interest). Opening and closing a short position is two trades, a sell then a buy. Just like a long trade there is no maximum holding period.
If you place your order to sell (short) 200 shares at $19, your initial investment is $3,800.
- If the stock increases to $20 your position is now at a loss of $200 ($3,800 - $4,000).
- If the stock decreases to $18 your position is now at a profit of $200 ($3,800 - $3,600).
In order to open your $3,800 short position your broker may require your account to have at least $5,700 (according to the 1.5 ratio in your question). It's not advisable to open a short position this close to the ratio requirement. Most brokers require a buffer in your account in case the stock goes up, because in a short trade if the stock goes up you're losing money. If the stock goes up such that you've exhausted your buffer you'll receive what's known as a "margin call" where your broker either requires you to wire in more money or sell part or all of your position at a loss to avoid further losses. And remember, you may be charged interest on the value of the shares you're borrowing.
When you hold a position long your maximum loss is the money you put in; a position can only fall to zero (though you may owe interest or other fees if you're trading on margin). When you hold a position short your maximum loss is unlimited; there's no limit to how high the value of something can go. There are less risky ways to make short trades by using put options, but you should ensure that you have a firm grasp on what's happening before you use real money.
The timing of the trades and execution of the trades is no different than when you take a plain vanilla long position. You place your order, either market or limit or whatever, and it executes when your trade criteria occurs.