It is very technical but there are computers that can place an order and cancel it in microseconds, fast enough to be able to 'see' the buy/sell depth off market, and not get caught having to buy or sell. The people with these computers have programs which then work out a buy/sell program which will enable them to make money on the following up or down movement of the share. Goldman Sachs had their program stolen at one point by an employee because it was so good and profitable. Previously banks and brokers had this inside information of clients buy and sell intentions, now merchant banks/hedge funds etc had this important information. Stock Exchanges allowed this to 'looking' to go on at a price. That is why a lot of trading is moving away from established exchanges to 'deep hole' trading places that are more private.
