Understanding time lag is a very important aspect of chaining success. When you create a chain, you cause pieces around you to shift spots. Sometimes, a match across the screen will occur at the same time that a hit of your chain connects. The game recognizes this as a combo. However, if the timing of those pieces falling is just slightly different than the timing of your chain, it results in an extra hit! Check out the movie below.

Most of the time, "time lag" occurs by accident.  However, with the right amount of practice, who knows....