How does 2-stage loudness work?

I found the vintage Grundig amplifiers have 5 tabs volume and I also acknowledged that that part is used for 2-stage loudness function. For a conventional 4 tabs volume, the loudness will proportionally vary with the volume level, or turning angle. I wonder how does the 5 tabs pot or the 2-stage loudness work? Is it equivalent to having 2 styles of equalization settings? I mean it won’t vary resistance with the turning angle, but have only two levels of bass/treble boosting? Do I understand correctly?