Where the term "short selling" came from is the stock market, for those who don't know.
An investor with tons of cash and stocks, and is certain a stock is going to go down, but wants to make more of a profit will borrow shares of stocks from others at a percentage of guaranteed interest + same stock payback. This method was perfected in the 1920's, and was a non-significant part of Black Friday.
Investor then sells all stocks, his and the borrowed ones (which, by definition are held long), causing the value of the stock to decrease. When the stock has decreased enough in value, the investor re-buys the stock at the new, lower price, having $dropped * shares in "Extra Money", which is more than enough to give back the borrowed stocks plus iinterest, as well as hold more shares than he did before, with money left over.
HSX does something similar, but acts as the "borrower" of those stocks, so when a stock is shorted, you borrowed a stock from the infinite HSX bank of stocks, and sold it, hoping to rebuy it at a profit when the price of the stock goes down. If the price of the stock goes up, the value is -$xxx,xxx, which means you still need to pay for the stock you "borrowed" at a lower price, but at the current market price, even though it is through the virtual HSX entity.
Thus, a large number of people holding the stock and selling, or a large number of people buying the stock will have a greater impact, as it is "Real Money" moving into/out of the stock. When you short a stock, you are using HSX's money, with the promise to "make good" on it later, therefore, it doesn't change the price nearly as much, since HSX has infinite shares.