This is the ability to quickly make a profit at the expense of short-term differences in exchange rates. That is, you choose, for example, the pair GBP / USD and CHF / USD (dollar as their common denominator), and then another couple and shared GBP / CHF (you can the find out its rate). First find out what the quotation GBP / USD currently is and what it is for CHF / USD. Now divide one by the other. Compare that number that you got from the current quote GBP / CHF: if yours is more then this pair will soon grow (literally within a few seconds), and if less, it will reduce. Earning through this is called currency arbitrage. It is called arbitrage, because you, as in an arbitral tribunal, need to monitor the everything at once and keep the whole situation under control.