Be sure to read the July Trader Magazine’s article on algorthmic flash trading, a/k/a front running:
Flash orders are also called “step up” or “pre-routing display” orders. The rationale for these order types is simple: Better me than you. They allow a venue to execute marketable orders in-house when that market is not at the national best bid or offer, instead of routing those orders to rival markets. They do this by briefly displaying information about the order to the venue’s participants and soliciting NBBO-priced responses. If there are no responses, the order can be canceled or routed to the market with the best price.
All four markets with flash orders treat these orders in a similar way. If they get a marketable buy order, for instance, that would otherwise be routed to a market quoting at the NBBO, they flash the order to some or all of their participants as a bid at the same price as the national best offer. Exactly who sees the flash, how that information is conveyed and the duration of the flash vary by market. The maximum allowable time for a flash is 500 milliseconds, or half a second, although most of the markets flash routable orders for under 30 milliseconds.
The details are worth a few minutes of your time.
Hat tip Bill King
Flash PointEquities industry clashes over flash and step-up orders
Traders Magazine, July 2009