I'm not sure I agree with your premise that large orders attract price. More likely, (as I believe Mark hinted), they are used to do the opposite. In any case, how would you trade it? Let's say you see large order on the bids, would you short by hitting the bid and wait for the price to move down to that order? How many ticks away does that order need to be from the ask(actually I was not clear if you are only checking top bid/ask sizes or multiple levels of depth)? What if that order goes away, do you immediately cover at the ask? I think this kind of strategy was used long time ago pre-hft and pre-anti-spoof rules.