“It was a dark pool version of front-running,” he said. “Say you get an order for something at 100. Immediately you go out and buy it for yourself at 100, in the hope that that will drive up the price, meanwhile not fulfilling the first order. If the price then goes up to 103, you sell what you bought, while telling the person who made the order that you couldn’t find a buyer. If on the other hand it goes down to 98, you fulfill the order at 100. Either way you’re good. There’s no way to lose.”
