The uptick rule is designed to prevent short sellers from jumping nonstop on a declining stock and driving it to oblivion. It was implemented in 1938 in the aftermath of the the depression and the huge market declines. Every bear market needs a scapegoat and the public loves to blame the villainous short-sellers for everything that goes wrong.
Short sellers borrow stock from their broker and sell it at the current price, hoping to buy it back at a lower price and returning the borrowed shares to the broker. Anyone with a margin account signs an agreement to allow the broker to lend out the shares in the account. In truth, you never know your shares are gone because the short seller has to pay you any dividends that are paid while he has borrowed your shares.