Money market funds are valued either on a "mark to market" or an accrual basis. The accrual basis means that every day one day's proportionate share of the coupons/interest is added on to the previous day's valuation. What this means is that if short term rates move higher then the accrual basis overvalues the assets in the fund (poor time to invest), and if short term rates drop lower then the accrual basis undervalues the assets of the fund (good time to invest).