Quote from Capablanca:
Call me an ignoramus but what are the NYSE closing imbalances?
The NYSE specialist must open and close his stock on a balanced print based on how many orders he has to buy and sell. The imbalance of these orders is posted in the premarket and 20 minutes before the close. The closing imbalances are also updated at 3:50 EST. Orders placed to offset the closing imbalances cannot be canceled after 3:40. There are traders following these numbers so it creates increased volatility/volume when the numbers are posted and updated.
I think you need to subscribe to the NY Open Book to see these numbers.
There is a thread called "Opening Orders - 2009" that has more info on the subject.