Could someone post a worked example perhaps? Including profit, loss and disastrous loss modes?
We will suppose that BTC has deflated to 1 BTC/USD
Suppose someone buys 10 BTC at 2:1 margin, it costs him $5 and mtgox pays the other $5. The $10 dollars is then sold at 1 BTC/USD. Then one of four things can happen:
* The order is fulfilled, mtgox takes a higher percentage of the transaction than normal because he provided margin (but not 50/50 because he took a lot less risk than the trader)
* The price of BTC in USD falls so mtgox automatically sells the order at the current market price, takes the money for his stake and passes the rest on to the buyer (who makes a loss)
* The price of BTC in USD can fall so fast that mtgox is not able to regain his lost capital (this is the only way that mtgox can lose money)
Mtgox has to sent his margin to a level that prevent the third point happening.