Please explain. Do you mean mortgage insurance? What are CDS?
Credit Default Swaps - not exactly "mortgage insurance" as most people who get a mortgage know it (Lenders Mortgage Insurance / Private Mortgage Insurance). CDS is a financial derivative that is an "insurance" (or a put option) against a default of portfolio or company. The higher the risk of issuer's default, the higher the value of related CDS.
It's somewhat complex, but understanding of the way it was used - combined with short selling - is at the crux of this financial crisis.
in particular, Defusing the Credit-Default Swap Bomb and How Short-Sellers Almost Destroyed U.S. Banking [System]. Also a discussion on Lehman's Chaotic Bankruptcy Filing Destroyed Billions in Value
The issue is, in real life how and why would one buy insurance worth several times the value of the property / portfolio? And what is the best and/or fastest way to benefit from this "insurance", i.e. make it pay? If you remember "viaticals" or "viatical settlements", think of CDS as viaticals on steroids issued against the life of financial institutions.