Imagine that you had arrived on the scene just as the banks were installing their first automated-teller machines (ATM) and watched as some customers used them.
Push a few buttons and, bingo, out comes cash. What a great idea. You watch it a few times and then decide to do it yourself.
Something analogous is happening in Hong Kong's financial markets at the moment. Traders at big American investment banks have for some time referred to the combination of the Hong Kong dollar, the stock market and the Hang Seng Index futures as their personal ATM.
They push the buttons by first working up a scare about the local dollar and simultaneously take short positions on the futures. As Hong Kong dollar interest rates then go up, they sell down a few index-sensitive stocks, say HSBC and HK Telecom, to help things along.
Bingo. The index goes down, they clean up on their short positions and the cash flows out of the ATM.
They have checked in at the ATM several times this year as the chart indicates. And now a few of the outsiders of this game want into it as well, which is why the latest bout of Hong Kong dollar selling features reports that it involves more than a few US banks alone.
It is a good time to play the game. Firstly, there have again been increasing predictions and rumours recently that the mainland will devalue its currency. It will be about the third time this year that the devaluation push has been on.