The thinking behind this strategy is to help you, as an investor, diversify your portfolio and increase the probability of higher returns. The long-term investments will provide the benefit of higher interest rates and increasing value over time. Maintaining some holdings in short-term bonds will provide you with flexibility to take advantage of interest rate changes.
When using this strategy, you'll want to keep your long-term bonds but be poised to make changes to your short-term investments (buy or sell as needed) if interest rates change. Evaluate your portfolio's performance and then consider selling and reinvesting your long-term investments when they reach approximately the half-way point to maturity.