[layoutIfNeeded]

They have servers in both locations. If you see that a stock starts falling in London you have a time window where you can still sell it at a higher price in say New York, until the price drops to the same level thus closing the opportunity for arbitrage.

[Consultant32452]

How quickly can you "see" the price falling in London and get that message to NYC? Whoever "sees" it first wins, right? That's what they're talking about.