Quote from wareco:
If rates rise, only those fixed rate mortgages are going to experience a reduction in MV. The value of the ARMs are going to ride the interest rate tide. Besides, a reduction in MV of assets does not create a "loss". It would create a reduction in net worth, but not a loss.
Someone correct me, but most of the retained portfolio consists of fixed rate mortgages, pooled as their own MBS. So ignore ARMs.
Suppose you have $10 in fixed rate mortgages, with a WAC of 10%. They're typically 30year, ie a duration around 6-10 yrs. You issue 1 year debt at 9%, and add 25c of your own capital. 1st year, net income is 12.25c. Year 2, oh horror, 1- through 10-year agency rates don't change adiabatically, but jump to 11% before you can say "buy more swaps." (or market says "sorry no swaps available today") Loss that year: 7.25c. By about year 6 or 7 your book equity is gone and you're insolvent.
Simplistic, but that's what being short funded means. Happened to FNM in early 80's I believe.
"Bankruptcy" is much more difficult, as it is more idiosyncratic. M-KMV build some (proprietary) trigger that's like "MV of assets 10% below book value of debt." Forbearance by the "regulator" (OFHEO) comes into play.
! Just tap the brakes on portfolio size (and growth), with some flexibilty for economic conditions (devil in the details: Bernanke could decide size limits from FOMC to FOMC
)