Nothing to do with flashers, much as that'd be a more interesting topic. Instead, flash trading happens when certain members of certain exchanges get a super-quick sneak peak of orders places on that exchange before those orders become known to the general public. This allows those folks to make trade decisions based on info that other folks don’t know yet. It’s basically the financial equivalent of saying, “Yeah, we were doing that before it was mainstream.”
If some people out there are thinking this sounds a little unfair, they wouldn’t be alone. Flash trading is controversial; opponents say it gives members of participating exchanges an unfair advantage over others wanting to trade on that exchange. Fans of flash trading say it’s not unfair at all, because it can enhance liquidity within that exchange. And that, they say, is always a good thing.