A volume pedal did it.

Yes, that breathing, sighing effect is just a volume pedal on a synth.

Richard Wright played a synthesizer (usually an ARP Solina String Ensemble or a Minimoog) with a volume pedal. He’d press down to swell the note in, then let off to let it fade. Each note breathes because he literally controlled its volume with his foot in real time. No tape splicing, no studio trickery — just a foot pedal and a good ear.

Sometimes the simplest technique makes the most iconic sound.