Hedge-fund manager William Ackman abruptly resigned from the board of directors of JCPenney—a company in which his fund, Pershing Square, holds an 18 percent stake. For the last couple of years, Ackman has been a controversial figure at the flailing department-store chain. He helped install former Apple executive Ron Johnson as CEO of the company. Since Johnson’s brief reign ended in tears, Ackman has been involved in a rare, highly public dispute with fellow board members over a replacement for interim CEO Mike Ullmann. At a July board meeting, Ackman had threatened to sell his stock and leave the company if a more rapid transition was not made. Now he’s making good on the promise. The big questions? When will Ackman unwind his stake in the company? And how many hundreds of millions of dollars will he and his investors lose?