from the online wsj http://online.wsj.com/article/SB100...990102886132.html?mod=WSJ_PersonalFinance_PF4 an interesting piece: "One such practice: 1,000 shares are offered for sale at $20, and someone wants to buy 2,000 shares at $20. The buyer should be able to purchase the 1,000 shares immediately, while the other 1,000 shares should instantly show as the new "best bid" at $20. Instead, says Mr. Narang, while the 1,000-share purchase goes through right away, the open order to buy another 1,000 is displayed to the entire market at a slight delay. Traders that can place orders faster can jump ahead, putting them in the best position to buy more shares at $20, in hopes of reselling them at a higher price. Mr. Narang says this occurs "at least tens of thousands of times per day." --------------------------------------------------------------------------------- and here's another quote that i just don't quite understand the mechanics behind. maybe someone else can explain: "Or consider a type of trading order called a "partial post only at limit." Here, if a fast trader's small buy order is rejected instead of executed, the firm can deduce that a large block of shares may lie hidden in reserve, poised to sell at a given price. Thus a trader may be able to get information without executing the trade. Clever use of this order type can increase the trader's odds of being in the right place at the right timeâcapturing a splinter-thin, lightning-fast profit before the institution can move."