Okay I think I know where your going.
I'm not litterally going like, moving my bum side to side out of the saddle or anything it's just like, trying to stay in rythm with Cheyenne's movement. I have a habit of posting, so I'm leaning forward, and doing all that kinda stuff, so when people ask me to show them my sitting trot, I'm not balanced enough to acually stay in the saddle, I keep on bumping around. So usually they tell me to go more in rythm with Shey-Shey's movement, like instead of forward and back it's side to side..... oh I don't know how to explain it. Just more moving with Cheyenne's quarters and how her legs go. Haha I'm a terrible explainer I know! (is explainer a word???)