This function is basically a slew rate limiter that reduces the speed of the output amplifier stages. This affects the frequency spectrum, the harmonics and the transient response at the same time.
Fast transients are slowed down a bit and the overall sound appears more round and merged. As this function influences the behavior of the output stages, the effect it creates has an impact on the complete processing results of all compression stages.
In this way the xpressor offers two different sound characters at the push of a button: the powerful transparency of the discrete class-A circuitry and the saturated richness of the Warm mode.