In his book Mortgage Wars, former CFO of Fannie Mae, Timothy Howard explains how Fannie's realization that mortgages behave like bonds with embedded call options revolutionized its ability to value its portfolio and manage risk. Prior to this change in thinking, Fannie Mae's methods for reserving capital were consistently shown to be inadequate. Today, the valuation of mortgages and mortgage-related securities as bonds with embedded calls is nothing new.
A call option is a type of derivative, which conveys the right (but not the obligation) to purchase another financial instrument (the underlying asset) for a specified price (the strike price) at a specified time (the expiration date). Purchasing a call option offers the right to purchase the underlying asset and selling a call options impose the obligation of delivering the underlying asset at the strike price on the execution date.
Mortgages are freely refinanceable at any point. In this way, they function as bonds in which the payments from the homeowner serve as the coupon payment and the ability to refinance serves as a call option sold to the homeowner by the mortgage holder. Typically the refinance rates increase as interest rates decrease. Although mortgage prepayment penalties are included in mortgages to discourage refinancing, a large enough drop in interest rates can make refinancing worthwhile to a property owner in spite of the prepayment penalty. For mortgage and MBS investors, prepayments are undesirable. Given that most mortgage investors look to invest anywhere between 5 and 30 years, an early decline in interest rates can leave many investors with cash from prepayments that must be invested in a market offering lower interests rates. This undesirable situation is the double-edged sword of prepayment risk for mortgages.