A broad reconstitution of the Russell Indexes occurs at the close of Wall Street trading today. This annual event will re-balance all the indexes in the Russell universe, but in particular the large-cap Russell 1000 and small cap Russell 2000. Those indexed to the Russell indices will have to follow suit at the close, meaning they will have to buy and sell the stocks going in, and coming out of, the various indices.
That's a tall order, given that there is roughly $5 trillion indexed to these benchmarks. Fortunately, the indices are weighted according to market capitalization, so it is not difficult for Wall Street to put together lists in advance of what will have the biggest activity.
The exchanges routinely hold a conference call at 3 pm Eastern (just before the market close), to monitor for unusual market conditions.