A market on close imbalance is an order the specialist has on hand to fill at the closing price. They come out at 3:40 and then at 3:50. You have to lock in your order by 3:53 and you can't cancel it. For example, T has a 3 millions sell imbalance come at 3:40, the idea is to sell the stock asap. At 3:50 the specialist will adjust the imbalance either to a higher amount or a lower amount in most cases, cause he may have filled the imbalance, or suddenly he may have a buy imbalance and you're dead. By 3:53 you have to send your MOC order to the floor, after that you can't cancel it, so the stock could move against you before the close and you could lose money even if it does pop on the close. So if the stockwas trading at $15 when the imbalance comes out, it may run down to 14.75 with all the people selling trying to satisfy the sell imbalance by buying back their shares on the close. But if there are enough MOC's put in, it could flip the stock to a buy imbalance or a much smaller sell imbalance and the stock could run up to say 15.25, and now you're out .50. At the close, if the original imbalance is still in effect the specialist could print it at $15, and you still lose .25. So it's not as easy as it sounds. There are specific strategies that need to implented to be successful, but just like regular day trading it take time to master. For example, some stocks are best left alone, and wait for them to pullback after the initial pop, others need to be chased cause they may keep going. Remember, there are 7 minutes in which you really have no control of the stock, and that's where it gets tricky.