It's all about balance on a slack line. I didn't do very well. Balance involves keeping your centre of gravity within the base area. In this case that will mean keeping the centre of gravity above the slackline. The problem is that the line keeps moving from side to side so you need to wobble your centre of gravity with it. I stuck out my arms. This increased my moment of inertia which makes it harder for me to spin one way or the other. That means there is more time for me to make the adjustment of my centre of gravity before I fall. It didn't work for me!
