Specifically, the ReplayTV looks for several sequences of all black frames at 30 second intervals. So, there's actually a delay of several minutes between when a commercial break ends and when it's identified it as a commercial break. Also, commercials that are longer or shorter can throw off the detection. Still, it works about 80-90% of the time, and when it doesn't, it's very simple to skip an arbitrary # of minutes ahead to compensate.