There is a name for this arm movement. (or other bod, facial, etc movement. I need to take notes in my sessions 'cause I've totally forgotten what the word is).
When this happens in my sessions the child parts are coming out, usually although more recently (the last year or so, it was a teenage part). Most parts have their own body movements that the therapist can now pick up on and it is essential for her to know them as she brings them all into the room together at times and welcomes them. I call it a bit Zen-like that the therapist can be so in-tune with me/us. It seems spiritual because I/we carried these parts without anyone knowing them for so many years but in actual fact they are so relieved to be welcomed into the present moment by outsiders that it is now becoming more like our feet are
planted securely on the ground like never before. The practical outcome is I/we are uniting in our internal community (Hive). Our boundaries are solidifying, our voice has more clarity. Yes, I/we do receive strength from our caring therapist but it is more like they are returning to me/us what was due in the first place. When we were robbed of our innocence and split off our energy was scattered too. That energy is being returned each time we encounter a new alter, work through the traumatic memory of that alter and talk amongst ourselves or play, as the children like to do.
At least to say, Writiing, that this is how we see what you have asked but this may not be what you mean. It is all so individual yet there are some commonalities at times too.
Love,
Hunny
.