Imagine being a retail foreign-exchange broker and letting your customers day-trade Swiss francs with lots of leverage. How much leverage would you feel comfortable giving them? Well, if daily moves are typically less than 0.1 percent, then that means that 95 percent of the time their positions will move by less than 0.2 percent in a day. So if you required 2 percent margin -- that is, you demand $2 of cash from them for every $100 worth of Swiss francs that they trade -- you'd feel pretty safe. That would mean that, 95 percent of the time, customers couldn't lose more than one-tenth of their equity in a day -- so if they lost money and skipped out on you, you'd be able to liquidate their positions without getting close to losing any of the money you'd lent them.
On the other hand when the euro/franc moves by 19 percent in a day, they're gonna get utterly smoked, and so are you. This is roughly the boat in which FXCM Inc. finds itself.
It's good to occasionally remember that a margin loan is a put: If you let your customer buy something for $100, and you lend them $98 of the purchase price, and then the price of the thing falls to $81, then guess what, you own the thing! Also you've lost $17. I mean, you can call the customer and ask for more money, it can't hurt. But you're not going to, like, feel full of joy and confidence while you're making that phone call.