DD has done this since she could sit up- she sits rigidly and balances on her bum (abs of steel) whilst her legs straighten and kick and hands sort of make little circles when she is excited. Mostly by In the Night Garden
Thing is, I have no experience of toddlers and the only time I have seen this behaviour is in children with ASD whom I have taught.
Does/Did anyone else's DC do this? Is it anything to be concerned about?