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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 - IQ4gui.
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.
You may find it easier to imagine the process as an automatic EQ, which can be used to cut a certain frequency only when it becomes a problem, or apply a boost only when that region isn't already loud enough.
While the audio signal passes through all four EQ bands in series, the side-chain filters are configured in parallel and are all fed from the main input, so changes to band one (for example) will not affect the amount of gain reduction or expansion applied by any other bands.