I've watched a lot of Irish dancing due to my daughters' participation in that activity over the years. A very quiet upper body is characteristic of Irish dance. All the action looks like it is happening from the waist down. The back, neck and head are held straight, and arms are held close to the body. It looks constrained to me compared to other forms of dance where the arms are more expressive. There are explanations for why this is so. One explanation is that dance was outlawed by the English rulers in occupied Ireland, so when groups of Irish got together some in the middle of the group could dance without giving themselves away to the English if they held their upper body rigid. I doubt there is much truth to that, since there is a great deal of jumping during the steps in Irish dance, so surely the English would notice heads pogo-ing up and down in the group, even if arms and torso were held rigid.
But what does this have to do with unicycling?
Today I did a 7 or 8 mile ride to the campus football stadium and back. I rode on pavement all the way. It was one of those days where my balance didn't feel too good, but regardless I worked on keeping my arms quiet at my sides, against my hips or clasped behind my back. I find that type of arm placement to be challenging and helpful at improving balance. Today while riding this way it struck me that one of the reasons for holding arms and torso stationary in Irish dance is that it really challenges and tests the dancers' balance. If you can keep your arms absolutely steady you really have excellent balance during your steps. Likewise, if I can keep my arms and torso quiet when unicycling, I'm well balanced and maintaining a steady pace regardless of the terrain.
I noticed from the video I posted last time that I didn't look all that steady riding down that trail on the unicycle even though at the time I thought I was under control. It was the active arms that made me look ready to lose control. I think if I maintain quiet arms and body when I ride, I give the perception of greater control, and don't scare pedestrians and others that I might weave into their path at any moment. So I'll keep working on hands behind the back, and improving my balance.