The Sound on Sound article about Ernst Zacharias notes that he "was supposed to be learning the organ" when he was young and that he built "his first organ keyboard in 1948." This may explain this bit of organ influence on the Pianet.
Swell pedals can change the dynamics, but they can't cut off the sound completely. As the user manuals for the early models (Mark 1, C, CH) explain: "Der Knieschweller erlaubt eine Lautstärkenregelung vom Pianissimo bis zum Forte." "The knee swell allows a volume from pianissimo to forte."
The Pianet N manual has this to say about the swell pedal: "The initial intonating effect when playing the Pianet may be practically eliminated to produce a smooth sound if the swell is jerked down briefly after the keys are struck. Conversely, with the swell full on, you can depress a key and sharply close the swell for a blunt, woody sound corresponding to the 'new sound' prevailing in the world of musical entertainment today." "Practically eliminated" but not "completely eliminated," so when Barry Carson writes in his Pianet N Background and History (which accompanies Nord's sample of the Pianet N and which I've posted here) that "volume was often controlled by a volume pedal, just as can be done on your Nord keyboard," it's not entirely accurate. When I play the Pianet N sample on my Nord Electro 5, I can completely eliminate "the initial intonating effect" and fade in the Pianet's sound from silence, but this wouldn't be possible on an actual Pianet. The initial striking of the note would be at least faintly audible.
As far as I know, the Pianet was the first keyboard instrument (with the exception of organs, of course) to have a swell pedal, long before volume pedals became commonplace with digital keyboards. I've seen complaints about the Pianet's lack of a sustain pedal, but I think the swell pedal is a more interesting feature. Like the Pianet's action of releasing a reed to make it sound rather than striking it, the swell pedal makes the Pianet a unique instrument.
