These transactions were very complex internally, but externally one can see quite simply that these select few constructed a money machine that took mortgages, a large percentage of which were junk, as input and put out mortgage backed securities that were rated AAA. Plus, these money machines insured the transactions as though the mortgages were top rated (a 2% default rate, as I recall).
FORBES: Hang it up, Wall Street. Dodd-Frank is here to stay