The practice of a mutual fund attempting to appear to improve its performance before it must report results to shareholders. For example, it may short sell losing stocks and buy gaining ones. It may also buy thinly-traded stocks with the intent of driving up the price. Portfolio pumping makes a portfolio look healthier (and, therefore, a better investment) than it really is.
The end-of-period trading by a mutual fund in order to raise the fund's performance results. For example, a mutual fund may purchase additional shares of a thinly traded stock it already owns with the intention of driving up the stock price and increasing the value of the stock in the fund's portfolio.