If we think a stock, index or financial instrument will go down in price, then, we can win this drop in the same proportion in which we could gain upwards.

This is a mechanism well known in the market called: THE SHORT SELLING.  It works this way: sell a share "now" to buy "after" to a lower price, just change the order of operations, first sell and then buy.

But where does the share that I will sell comes from, if I have not bought it yet? Then the broker will lend it from another customer who had already bought it, but the money from the sale cannot draw from your account until you do not return the share to be, but you can use it to buy shares, and once you returned the shares you keep the difference.

Example: You request a Microsoft’s share that is now worth $ 21 dollars, you sell it on the market.  So you now have $ 21 dollars in your account (you can not remove), but you owe a Microsoft’s share, and note, you DO NOT owe money, you owe a Microsoft’s share.

In a month, the Microsoft's share goes down $ 15 dollars, from the $ 21 dollars that had you take 15 to buy the share and you returned it to the broker, leaving you with $ 6 dollars gain (21-15 = 6).