As far as I remember from SD1, swell is ment as setting for the volumepedal, and when choose i.e. to control only the up and down volume for right hand voice, it did exactly that.
At first it controlled all outgoing sound as a main volume switch on my SD1, but when post a question to find out where to navigate to do the settings, everything worked out well.
I never found that this feature does anything else than make you rise and lower the sound volume of parts choosed in settings.
Cheers and Happy Playing (as well as Happy Easter)