Most of the systems we have seen allow complete fulfillment of a BUY or SELL order simply by clicking on a market order that appears on the screen. The Nasdaq system then gives this market order to the daytrader. We say presumably because it may have changed in the short time period since the buy, or even in reaction to the daytrader's buy. It would seem that the ability to buy and sell stock in between the spread would guarantee profits. But two things make it more difficult than at first glance.
Transaction costs make it necessary to operate at high volume orders.
But, market shifts make it hard to move large volumes at a single price. This is just to recover the transaction costs. To make money requires a much higher share volume. Of course, the daytrading firm's systems can also be used to place ordinary limit orders just as other investors do. But this is like using a shotgun to swat flies.