My 5yr. old nephew wiggles his fingers very rapidly in front of his face when he's excited. It could be something as simple as turning a page in a book and seeing a new picture.
The other day at a museum,one of the exhibits was of balls going up and down in a maze. He couldn't keep still and kept jumping up and down while at the same time doing the hand thing.
I couldn't help observe how other children, some younger than he, were enjoying it too, but not doing what he was. It's as if he can't control himself. Is this just something quirky that kids grow out of?