Its not about trying to do specific things you just sort of go with it to be honest. I don't really know how to explain it but when you see dressage riders doing trot they seem to fit the horse perfectly and sort of nod their head in time with it - they don't resist it. You shouldn't be trying to use knees, thighs or whatever - just go with the movement. Just totally sink into it - at first if it helps forget about trying to keep good upper body posture - just totally go with it - sink into it. Once you have mastered the trot without bouncing so much you can then worry about what your posture is like.

DT your friend is a couragous lady to have gone on to do very well in the horsey world after brain damage