So you mean buying BitShares equals short BitAsset?
If so, assume there are 32 kinds of BitAssets and each is pegged to different asset, then what is the rate?
They are market pegged each BitAsset class floats against BitShares based upon supply and demand factors alone. Of course BitUSD can only be borrowed into existence and is destroyed when the loan is repaid to the blockchain.
The total bitshare is LIMITED, so I wonder your backup will be insufficient unless 1 BitShare can support multiple BitAssets.
Suppose a BitShare has value worth $100
1oz of Gold is $1000
You can borrow 1 BitGold from the blockchain provided you post 20 BitShares (worth $2000) as collateral on your loan. This will create new BitGold and you will be unable to spend your 20 BitShares or receive dividends on them until you pay off your loan. If the value of 1 BitGold rises relative to BitShares then your collateral will be used to repurchase 1 BitGold and pay off your loan and give you back what ever BitShares are left over... likely 5 BitShares. When the loan is repaid the 1 BitGold is destroyed.
So, the only way for X value worth of BitGold to exist is if 2x value worth of BitShares already exist. The only way to increase the supply of BitGold is for the value of BitShares to rise enough to provide the proper backing.
This is exactly how 99% of real USD is created. First there must exist 1.2x value worth of House. Then X USD is lent into existence. When the loan is paid off, X USD is destroyed.