Imagine a standard compressor with a filter inserted in the side-chain,
so that it only responds to certain frequencies. Now imagine that
instead of controlling the overall gain of the signal, it only controls
the volume of specific frequencies.. you just discovered dynamic EQ! In
other words, it can compress or expand specific frequencies in a similar
manner to multi-band compression (but with a very different character)
or it can de-ess, or perform more esoteric functions such as ducking one
frequency when another is prominent.