Late-Day Trading

What Does Late-Day Trading Mean?
An unethical (if not illegal) practice of a hedge fund purchasing and then selling securities (usually shares of a mutual fund) after the close of a trading day, but making the transactions appear as though they occurred before the market close.

Investopedia explains Late-Day Trading
For mutual funds, net asset value is (NAV) determined at 4pm EST (the market close), and it does not change until the market opens again. Hedge funds involved in late-day trading work out a special relationship with a mutual fund that, usually for higher-than-average fees/commissions, allows the hedge fund to buy and sell mutual fund shares after hours but record the trade at 4pm. This practice gives the hedge fund an opportunity to profit when material information affecting the fund is released after the market close. In such cases, because it is stagnant, the NAV may not represent the actual asset value, which won't materialize until the market opens again - at which time late-day traders sell their shares at a profit.