CDSs are a perfectly reasonable construct. It's just an insurance contract for credits. If I give temestk a credit of $1000 with 10% interest to be paid back after one year and then I have second thoughts about you actually paying the full amount or anything at all, I can ask around. Who knows temestk? Who would pay me whatever he fails to pay me and at what rate? Now your bank that knows you well might say "Cool, free money for us. A temestk always pays his debt! For $5 we guarantee your full payment."

As you fall behind on your mortgage payments, the bank now has second thoughts and would like to get rid of this risk with a $1100 down side and searches for somebody else to cover it. Your brother might accept it for $15. The bank made a loss of $10. Your brother is on the hook if you fail to pay.

So yes, it can get convoluted and hard to track but the price to secure the debt is a valuable indicator and I think it should be public but of course, the bank trying to sell and you don't want this to be public.