Each channel of the hardware alpha compressor features an additional transformer that can be switched into the signal chain after the mix stage. These are classic output transformers, but they are not used for balancing and galvanic isolation, but as an additional means of sound shaping.
We have decided to approach this in a little different way with the software version. Here you get something similar to the Warm function known from the museq: a slew rate limiter which alters the frequency spectrum, harmonics and transient response.
Because of the mastering approach of the alpha compressor, this is much more a subtle audio shaping feature than a glaring sound effect, but it's certainly nice to add a little bit of color to a signal which might otherwise seem to be too clean.