The movie shown below, developed by a real-time trading software developer Nanex, shows the stock trading activity in Johnson & Johnson (JNJ) as it occurred during a particular half a second on May 2, 2013.
Each colored box represents one unique exchange. The whote box at the bottom of the screens shows the National Best Bid/Offer, which often drastically changes in a fraction of a second. The moving shapes represent quote changes which are the result of a change to the top of the book at each exchange. The time at the bottom of the screen is Eastern Time HH:MM:SS:mmm, which is slowed down to be able to better observe what goes on at the millisecond level (1/1000th of a second).
In the movie, one can observe how High Frequency Traders (HFT) jam thousands of quotes at the millisecond level, and how every exchange must process every quote from the others for proper trade through price protection. This complex web of technology must run flawlessly every millisecond of the trading day, or arbitrage (HFT profit) opportunities will appear. However, it is easy for HFTs to cause delays in one or more of the connections between each exchange. Yet if any of the connections are not running perfectly, High Frequency Traders tend to profit from the price discrepancies that result.