So, I wanted to make proper animations that add actual bouncig butt (not imitation through breast bones) and I had a lot of hard time without any result.   Thing is that I add correct animation with pose converter feature, my mesh is correctly rigged to needed bones. Everything looks fine in Blender and in Nifskope. Buttock bones move and rotate as I said them to. But there is nothing ingame itself. Butt is not animated, while tiddies work.   I checked lowerbody mesh a lot o