These costs are incurred when the execution of an order moves the price of a security above the target price for a buy order (or below the target price for a sell order). Dark pools help to minimize information leakage about a large order before it is executed, whether through broker chatter or otherwise (the forces of supply and demand generally work to move prices down in the face of a large sell order and move them up in the face of a large buy order, so by hiding order size a buyer or seller my get better executions as a result).