I found a halfway decent summary of MBS, CDO, CDS -
https://medium.com/@danwwang/the-cdo-the-cds-and-the-subprime-mortgage-crisis-c1aa28c01116#.qlex0dbdx
A key point that is not really discussed is the fact that the CDS contracts were bilateral contracts between, say, Scion Capital (burry, portrayed by Christian Bale) and Goldman Sachs.
Scion, as the buyer of the contract, is paying on an insurance policy with Goldman, where he would receive a payment to make him whole on any losses taken following a specified set of circumstances (outlined in the CDS contract) in the underlying security on which the policy was sold. Typically, on a classic corporate bond CDS contract the insurance payout would only be triggered by a default of the underlying corporate bond, and the insurance payout would be determined by the fate / market pricing of the underlying corporate bond following the default. Default is a specific legal condition that would have been spelled out in the indenture of the underlying security - most obviously a missed interest ir principal payment or bankruptcy of the underlying borrower.
We can assume that it is the same here, where Scion would onlg get paid off on the long CDS position (short mortgages) following the default of the underlying security. If the underlying security was the BB or BBB tranche of a mortgage backed security that contained a significant concentration of subprime mortgages, it would not have taken a lot of people to stop paying their mortgages to massively increase the likelihood of default of the junior tranches (i.e. Not AAA, AA, or A) of that securitization, but even the BBBs and BBs would have been reviving enough income to make interest payments (maybe 6-7%) as they came do, based upon the relative sizing of those pools.
So, even though Scion was absolutely correct, and the defaults began building in the securitization tranches that they had insured in their CDS contracts with Goldman, in the absence of actual default of the pool tranche itself the specific pricing ("mark") of their insurance contract was whatever Goldman said it was. There was no exchange that they could look up MBS CDO CDS spreads. They just had an insurance contract with a party whose interests were not aligned.
The notion of their pricing being "fraudulent" is debatable - if there were willing sellers of the CDS at prices sustainable by their marks, despite the pricing and performance of the underlying securities, they would have had a lot of latitude in this area. When the 2 internal bear stearns hedge funds were blowing up in early 2007, there was an article in the Wsj that outlined a potential plan of theirs to actually begin buying some of the houses in the underlying pools in order to prop up the marks. Obviously it didn't work.
Early on, it is almost assured that the counterparties on the other side of the scion CDS contracts were the banks themselves. It was free money to them until the very end, when actual pools would,have defaulted. Then the banks began offloading this risk to other institutions - with AIG being the most prominent. As the default probabilities increased and the mark to market values began moving in the favor of the Scions (and the banks were now on the same side of the trade - the bale phone call with Goldman before it moved in his,direction), all of the sudden the cash had to move in the other direction on the contracts, creating massive liquidity needs. AIG getting downgraded itself was a major precipitating factor. So instead of actually collecting on the insurance policies, most of the shorts would have sold their positions and cashed out.