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.
Thank you for your answer.
Just bear with me a few more, please.
Assumption: BitShares has non-0 value.
Therefore there exists a ratio between USD and BitShares, lets assume the initial condition is $1 per BitShare
Mr. S Owns 1000 BitShares and believes they will go up in value relative to USD
Mr. L Owns 1000 BitShares and believes they will go down in value relative to USD
They enter a transaction on the block chain that takes 2000 BitShares as input and creates 2 outputs.
Mr. S receives a BitUSD short Position that allows him to Recover 2000 BitShares provided he buys 1000 BitUSD.
Mr. L receives a BitUSD long Position that he owns until he chooses to sell.
Time passes and the price of BitShares vs USD goes to $2 per BitShare. This means that Mr. S was right to short USD. Mr. S now believes the price is too high so decides to exit his short position. To do so he must buy 1000 BitUSD on the market which now costs 500 BitShares. He places a bid in the market, when it is accepted 1000 BitUSD is destroyed, Mr. S walks away with 1500 BitShares and the seller of BitUSD walks away with 500 BitShares.
In an alternate reality, time passes and the price of BitShares vs USD goes to $0.66 per BitShare and Mr. S's collateral will only cover 1500 BitUSD so a miner performs a margin call by using the 2000 BitShares held as collateral to accept the lowest ask $0.65 per BitShare and as a result Mr. S is given (say) 462 BitShares while the seller of the 1000 BitUSD is given 1538 BitShares.
What we can conclude here is that even though no USD changed hands, Mr. L retains the purchasing power of $1000 USD regardless of which way the market moved. Mr. S assumed the risk of BitShares going down in value, while Mr. L assumed the risk of BitShares going up in value. By exchange risk exposure the result is the creation of an asset (BitUSD) that is market-pegged to USD.
So how do we know BitUSD will be market pegged to USD? Because it can only be created when a long and short agree on the ratio and they have opposite incentives. The Long wants as low a price as possible while the Short wants as high a price as possible. How do they know that what they are trading is really a bet on USD other than the Name? Because the market depth will establish market consensus as the bid/ask spread starts wide and gets narrower until there is enough depth that someone is willing to make the first trade.
How do we know that the price will track? This comes down to making a prediction about how future market participants will judge BitUSD vs BitShares and the only rational way to make this prediction is to assume future actors will bid according to the future USD vs BitShares. The price tracks for the same reason that centralized prediction markets can price the probability of an event.