It works by rotating the crown. I was a bit concerned that it would be over sensitive and whack the volume up with just a nudge. But it’s actually well implemented and it’s quite easy to increase or decrease the volume just a little. A visual indicator appears while you’re doing this.
UPDATE: Naim has not responded directly to me but they have certainly corrected the problem. Following the latest update to the Focal & Naim app I can, by turning the Digital Crown on my Apple Watch, control the volume on my SN3 via the System Automation connection to my NDX2. Thank you Naim.