 If interest rates go down, you have some longer-term bonds that'll go up in value a lot quicker. And when the shorter-term rates start to mature, you're only reinvesting a little bit of your money at a lower interest rate. Conversely, when rates go up, your long-term bonds drop in value faster, but you have your short-term bonds come in due, so you can reinvest that money at today's higher interest rate. So the idea is it's a way to hedge your bets with interest rates in the bond market.