I'm still trying to understand that. If I understand well, when you go long on an asset, it's basically as if you were using borrowed money to buy larger volumes of that asset than you'd be able to do with your money. If this asset goes up in price, you are able to pay your debt and also make a larger absolute gain. But if it goes down, your "reserve" will be burn out quite quickly. For ex. in a 10:1 leverage, a -10% oscillation would force your position to be liquidated so your debts could be payed.
Now back to BitUSD.... how is that a long position in USD? Isn't that a coin issued by someone? Anybody can issue more BitUSD as long as they deposit the necessary reserves in BitShares? So, let's say I believe the price of USD will go up in relation to BitShares, how do I proceed? I set aside, say, $1k worth of BitShares on a 10:1 long position and then I can issue myself 10k BitUSD? If the value of the USD goes near 10% down in relation to BitShares, miners would be able to confiscate my reserves of BitShares and destroy the corresponding BitUSD, I assume. But how would miners decide which is the "protocol-official" price of USD in relation to BitShares? Do you intend to create some price publishing mechanism like those in Mastercoin, and each BitWhatever issuers would decide which publish(er) to use?
Not anyone can issue BitUSD... the only time BitUSD can be issued is when a long and a short agree on a price. Each posts an equal number of BitShares resulting in a short position with 200% collateral and a long position. If the price moves against the short, the collateral will be used to repurchase the long position on the open market (all BitUSD is fungible) and the short position loses money. If the price moves against the long position then when they sell their position they end up with less BitShares than when they started. It would be like selling BTC for USD and then watching the price of BTC jump $30 in one week when you repurchase BTC you end up with less than you started.
The long position maintains purchasing power equal to 1 USD, but no USD ever needs to enter the system or be held by a trusted 3rd party.