How does a vari mu compressor work?
A variable-mu (variable gain) compressor is an analog compressor centred around a vacuum tube. As the input signal increases, the current sent to the tube's grid decreases, resulting in a reduction in the overall level.
Are Klanghelm plugins good?
This is an amazing plugin, it features 3 different compressors for a very cheap price. There's also a free slimmed down version for you to try out, but I would get the paid version. It has replaced more expensive compressors for me, absolutely stunning quality. Highly recommended, and for that price it's a steal.