Could you describe and explain how Michael Burry using the credit default swap in "The Big Short" film to profit from the collapsing house market? What is the underlying asset of credit default swap in "The Big Short" film? Why Michael Burry and the others sell the credit default swap to close their position before the maturity date? Why they dont wait until maturity to receive a payoff if they are sure most mortgage contracts will be default triggered by the housing bubble?