If you convert HBD right now you would receive around 73 cents worth of HIVE. The market "knows" that hence the current price. The blockchain rules will only backup the value of HBD up to a certain point (max 10% of the total HIVE supply).
If the value of HIVE goes below that threshold then HBD will be worth less than 1 USD. If the price goes above one dollar then the market is being stupid (since the blockchain will only give you maximum 1 USD worth of Hive if you convert it).
There is no such thing as a stable currency but HBD should follow the price of one USD in the long term but we should expect fluctuations in the short term.