Your BitUSD stays your BitUSD forever, until you put it on the market. The person who is forced to cover will buy someone elses BitUSD and burn them instead to lower the supply. In a black swan event the price will be falling so quickly that there isn't enough BitUSD on the market to cover the amount of BitUSD you originally bought from him. In that case he will buy as much as he can on the market with his collateral and burn it, and then the yield fund will burn the remaining amount. If the yield fund goes dry, BitUSD becomes undercollateralized - what that means is that all future yield will be burned until BitUSD becomes properly collateralized again.