Without enough flare the bar will be digging into that shoulder. Rocker is the curve of the bar from front to back. A decent rocker is essential so that the bars do not bridge and therefore rest on single points front and back. This can create sore spots as the weight is borne on individual locations rather than spread along the length of the bar as it should. This is also a point to consider when riding a mule.
Mules have backs that are much straighter than horses' backs.
So a horse saddle with good rocker will actually be pretty distressing to a mule. It will actually rock forward and backward on a central point. Just as a tree without sufficient rocker will be painful to a horse from bridging, this will sore a mule's back from chafing and weight bearing at a central point. Twist refers to the way the bars turn as they go from front to back allowing for the tree to rest alongside the shoulders up front and on top of the loins at the back. If the bars do not twist sufficiently to rest on top of the loins then they can be digging the bottom edge into some hefty muscle back there.