Why Slow Light from a Wavelength-Dependent Wave Speed?

To understand how a wave speed that depends strongly on wavelength can lead to slow light signals, Welch suggests imagining a pair of overlaid combs. If the combs travel at slightly different speeds, the group velocity of the patterns they create can be dramatically different (faster in this case) than the speed of the combs themselves. The wave mathematics are also similar for two sound waves that have slightly different frequencies--the combination generates a much slower wave with the so-called "beat" frequency.