Found this article The large Blocks that trade at 4pm are often NOT placed at 4pm. What you are seeing and referring to, is the closing print in a stock where orders are placed to be traded on the exchanges’ close. Rules have been put in place to curb volatility and the manipulation of stock prices that force you to place orders by 3:45pm on the NYSE and 3:50pm on the NASDQ. Because of these rules and the fact that regular trading for the day ends at 4pm, many orders aggregate to make up the large blocks you see trading at 4pm. Large participants in the trading of stocks, such as mutual funds, hedge funds, sovereign wealth funds, etc. with the exception of corporate insiders, look to this time of the day to access liquidity in the market that naturally comes with the close of the stock. In the case of large imbalances to buy or sell on the bell, the exchanges will announce to the news wires these imbalances. This too is done to curb volatility at the close in that it is seeking off-setting bids or offers which can be placed after the 3:45pm and 3:50pm marks respectively. If you wish to trade as part of these block transactions you can enter an order as “market on close” (moc) or “limit on close” prior to the times outlined above. NYSE floor brokers have tools at their disposal to access the close with different order types. Unfortunately, due to the evolution of electronics in the market place, it is usually only large institutions that use the skill and knowledge of the floor broker to access the closing print with the tools mentioned above
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