Considered an unfair advantage in stock markets, “flash trading” appears to be on the way out, thanks both to new federal regulations and voluntary decisions by leading exchanges. The Securities and Exchange Commission proposed new rules on Thursday that will ban the practice involving high-speed computers that allows traders to buy and sell stocks at a fraction of a second to determine—and manipulate, critics say—the direction of trading, issuing and canceling orders to determine the price limits of buyers and sellers. Flash orders are part of a larger practice known as high-frequency trading and represent just a small part of the overall stock market business (less than 3% in July).