Probably a digital swell pedal.
On actual pipe organs, one of the keyboards (usually known as the "swell") has all of it's pipes in a big wooden box, with something like a venetian blind bolted on the front of it... and the slats on that swell box can be levered open or closed using a mechanical pedal, thus controlling the volume.
(, Sat 10 Jul 2021, 21:13, Reply)