I know it sounds bizarre but it seems its a thing!
www.reddit.com/r/autism/comments/r4vd7i/anyone_else_here_randomly_put_your_hand_in_the/?utm_source=share&utm_medium=android_app&utm_name=androidcss&utm_term=1&utm_content=share_button
wrongplanet.net/forums/viewtopic.php?t=32049
My 19 year old diagnosed DS asked me the other day if I ever put my arm up in the air when I'm lay down in bed and I do! It feels so comfortable up there. I mainly use my none dominant hand and can do it for quite a long time.
Today I noticed that my 8yo DS who is waiting for his diagnosis does it too 
I've no idea why we do it, it's obviously some sort of stim.