This function emulates the effects of an analog 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 EQ stages.
This way the museq plugin offers two different sound characters at the push of a button: Powerful transparency by default, and the slightly colored richness of the Warm mode as an alternative.
Please keep in mind that this is much more a subtle audio shaping feature than a glaring sound effect, but it is certainly nice to add a little bit of color to a signal which might otherwise sound just too clean.