Not sure is it common or not, internet articles made me worry since they say it can be first sign of non-standard neuro development.
He does it when excited and last days it is noticeable since he does it with both arms. For example seeing new toy first time he will hands flap and tense up for 4-5 seconds.
He does it only when excited or frustrated, not during usual activities.
Before 16 Months of age he was arm twisting sometimes when excited, but not flapping as now.
Any similar neurotypical case?