Since the CBOE’s introduction of Weekly options, our opportunities to take advantage of expiration day dynamics have quadrupled for many stocks/indexes. Depending on your position, it is fascinating/horrifying to watch the premium collapse on options in their last couple hours of trading. Jeff Augen in SFO weekly has written an interesting article (New Approach to Trading on Expiration Day) on some of the underlying mechanisms behind the last day dynamics.
He points out that when a stock moves above a strike price (e.g., 380 for AAPL) on expiration day, covered call writers with just now in-the-money calls are motivated to close out their positions, as well as those long the calls. The bid price on the options can be low enough that large scale option holders often prefer to exercise their options and sell the stock to close out their positions. The combined selling pressure from these processes can mute, or even reverse an upswing in a stock.