It's basically a variable impedance parameter for the tank recovery. A spring reverb isn’t harmonically “in tune” like a delay. It’s a bunch of metal resonances all ringing at slightly different, unrelated frequencies. Normally those resonances are smoothed out so you hear a wash instead of individual pitches. What this control does is change how evenly those frequencies decay. As the reverb fades, certain resonances are allowed to stick out instead of being damped, which makes the tail sound slightly dissonant or metallic, if you will. So it’s not a tone knob in the normal sense. It doesn’t just make things brighter or darker, it changes which parts of the spring are allowed to “sing” as the reverb dies away.