It is the pose of a person who bye bye with his whole body.
It can also be used in other situations, such as when you find someone in a meeting or crowd, or when you hurry up and say wait.
全身でバイバイする人のポーズです。
他にも待ち合わせや人混みで見つけたとき、慌てて待ってと言うときなどの場面にも。