Quote from Rearden Metal:
Oh, I already assumed that part!
Here's what I think might have happened:
Customer calls shady broker at 10:00 AM with an order to buy XYZ at market. Shady broker easily fills the order immediately at $100, and waits to see what the market does in the next few minutes.
If by 10:06, XYZ is trading around the same price ($100) or lower, the original prompt fill is assigned to the customer, and that's the end of the story.
If by 10:06, XYZ is trading all the way up at $102, Shady broker keeps the original prompt fill for himself (or his proxy, to make the scam's paper trail harder to detect) and sells it to the customer six minutes late at $102, pocketing the $2 difference as a risk-free arb. This is illegal, of course, and the broker has to have very little respect for his customer/victim's mental capacities to try and pull a douchebag stunt of that kind.