From The Economist, 3 January 2009:
The second feature of the Danish system is that mortgage-holders can also buy the bonds in the market and use them to redeem their mortgages. This is useful if a rise in trest rates (or a fall in house prices) causes mortgage-backed bonds to trade at a discount. Redeeming their bonds allows homeowners to reduce the amount they owe. In America, for instance, mortgage-backed securities have fallen far below their fundamental value in thinly traded markets, partly because the people who would benefit most from buying them have no mechanism to do so. “Everybody can buy that bond at a discount except that one guy who is most involved with the loan, the homeowner,” says Alan Boyce, a mortgage expert who has worked with George Soros, an investor and philanthropist, on promoting the Danish model in emerging markets. In Denmark, by contrast, a fall in the value of mortgage bonds usually encourages homeowners to snap them up to redeem their own mortgages, as is happening now.
Seriously: Do not trust anyone who uses the term “tranche.”