Reading through examples of legal front running, I'm struggling to understand how "penny jumping" (http://www.wikinvest.com/wiki/Front-running) can be profitable.
Suppose stock ABC is trading at a spread of \$50.1 - \$50.9 and trader Alice has placed a very large day buy order at the \$50.1 level. Trader Bob who is supposed to profit and limit his risk is placing an order at \$50.2, one tick up.
Now your typical description would say something like: "if ABC's price rises above \$50.2, Bob will profit, but if it doesn't he'll be able to sell for \$50.1 as part of Alice's large limit order."
Down: This is a buy order, so if the price goes down it is because there is sell pressure, and sellers are eventually crossing the spread. When Bob's buy order has traded he can still scratch trade by selling back against the impenetrable $50.1 bid level, and he didn't lose much. His risk is indeed limited.
Up: (Here's what I don't understand.) If the price goes up it means more bidders are joining the queue on the bid side pushing the levels up above \$50.2 where Bob's order resides. Bob's order will not get done at all - where's the profit here? We'll simply discover by the end of the day from the average close price that "ABC went up".