Ok, to sum it up - flapping may be both or just one hand/arm. Thanks for the insight! (We can consider this question closed - no need of further responses. Cheers.)

it is definitely a self-stimming behavior. My daughter eventually learned to cross her arms to hold it in... have to say sometimes I miss it, it was adorable to me.

My son flaps one or both hands when he gets excited...that's sort of his way of smiling!

My 8 yr old son would vary on his flapping a.k.a "stemming". Sometimes it would be one and sometimes two. He rarely does it now,but if he get's too frustrated or excited then look out cause they will flap.Keep letting him scribble with the pens while supervised because no matter how much he attempts to eat them try different techniques. I would do a lot of hand on hand guidance when trying to encourage my son to scribble because he had finger dexterity issues and could barely even hold a pencil. Now I can't keep one from him.As he is an excellent artist.

My daughter flaps when she is excited and happy. Sometimes she only flaps one hand, but usually it's both. She also sometimes holds her right hand up in front of her face as a "filter." She is also four and a half. As she ages, the flapping is becoming less and less.