HoRNet tells us that Magnus is a 2 stages brick-wall limiter, designed to make your tracks the loudest possible. Here's their own description of its operation
To achieve making your tracks the loudest possible the limiting process is divided in two steps: the first is a clipper, hard or soft, that let you cut away those small peaks that nobody is going to hear, the second is a limiter with a fixed threshold of -0.2dB.
Simply use the two big knobs to increase the level until you are getting the right amount of gain reduction by any of the two stages.
Do you have a very percussive track? Use more clipper to cut those sharp transient and let the limiter work less. A soft song? Abuse of the limiter and don't use the clipper.
The limiter in Magnus is a quite sophisticated tool, the attack it's instantaneous and the release is made by two different stages, the first one is very short and the second is longer so that the plugin is able to recover fast from sudden peaks but able to keep a sustained level to avoid obvious pumping.
Going into technical details, the release time set with the release knob is the time of the longer envelope, the short one is exactly one fourth, so that it will always musically match the tempo of the song.
If you are unsure, use the Auto Release button that sync the release time with the song tempo reducing pumping as much as possible also when generous gain reduction is going on.
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The long time pedal manufacturer takes the first steps into the modular realm