This "formula" was what big banks used to solidify their mortgage backed securities 12 years ago. Instead of letting houses default, they would buy them up.

So imagine you're BoA and in one town you back 5000 mortgages. Let's say 500 of those are about to default. You've already sold "paper" that includes all 500 of those loans, some of which may have been in the AAA tranche. In order to keep your AAA tranche from collapsing, you buy those 500 houses outright and sit on them, providing the illusion that your securities are healthy. It's cheaper and simpler.

The downside is, when the market collapses, BoA is still stuck with 500 houses and will have to sell them eventually. They don't care. They're just trying to hold on to them until the securities they sold mature. Once that happens, all bets are off and they dump the houses into a depressed housing market.

Housing prices are going to collapse. Badly. You can bet on it.