Consider a bond that has a single cash flow one year in the future. The yield curve is steeply sloped upwards at the 1-year maturity point, but flattens out at longer maturities.
Suppose that market conditions do not change for a month. At the end of this month, this one-year bond will have become an 11 month bond, and the yield used to price this security will now be read from the 11 month point rather than the 12 month point on the yield curve. Since the yield curve is downwards sloping, the 11 month yield will be lower. Since the yield is lower, the price will be higher, and a positive return will have been generated.
This strategy is sometimes called riding the yield curve, as it is most effective when a security’s cash flows are positioned at maturities where the curve is most steeply sloped.
Note that this return has not been generated by movements in the market, since we explicitly assumed that market conditions were unchanged. Nor has it been generated by elapsed time, because the return is generated entirely by a change in yield. Roll-down is therefore distinct from either source of return, and should be measured separately.
Use the RollDownAttribution flag in the configuration file to indicate whether roll down return should be shown on attribution reports. If not shown, roll down return is added to the residual return for each security.