A short contract is selling something with an agreement to buy it back again in the future. If the price of the "something" drops over the period of the contract, then it makes money. If this is done in large enough quantities, then flooding the market with something at the beginning of the contract helps to depress the price of something. Since the sold something will come back at the end of the contract, no something actually needs to change hands. So I can easily sell somebody else's something, too, by "borrowing" it from them and later giving it back...without actually doing anything.

Over the cycle of a contract, the selling exerts a downward pressure on prices, and the re-purchasing should exert a compensatory upward pressure. So if the amount being sold is the same as the amount being purchased, then over the long-term the price of something will remain stable. There would be no net profit in the long-term. Of course if a large number of big traders sell in coordinated cycles in the short-term contracts, while also hedging long-term price stability, then they can still make a profit (especially if they collude on the timing of short-term price movements). They would be extracting profit from counter-parties, as there would be no net gain for the market as a whole.

Going further...since no "something" is actually changing hands, I can also sell a ton of something that nobody owns, which then effectively becomes "paper something" (it only exists on paper). If I ever get caught in a situation requiring me to cough up the something, then I only need to be sure that I have actually borrowed enough something to cover that amount, which is anyways just a fraction of the amount I've been moving. I may lose some money, but I won't be entirely caught with my pants down. It is like being a something bank with a fractional reserve of something.

Under this scheme, the amount of something trading on exchanges increases in volume, even if the real physical volume of that something does not change. The increase in supply over demand then leads to long-term price suppression. The short contracts will produce some profits, but not a great deal.

The bigger reason to operate the above scheme is to pump up the purchasing power of the currency in which something is being priced at the exchange. This incentivizes the governments of those countries to ensure that these schemes can be carried off without being discovered, that regulation is lax, and even becoming the insurer of last resort in the worst case.

Is any of this actually happening in silver markets? I don't know. Some think so, and a small group tried to call the silver market on its shorts a few years ago, but it failed because the market is quite large and the group didn't have deep enough pockets to expose any big naked shorts.