Whale yoga!! Only twice in my life with cetaceans have I seen whales 'stretch' like this. Once with a Fin whale who surfaced and literally stretched, simultaneously showing a rise of the head and dorsal fin. And this individual Humpback whale that was lobtailing next to our vessel for several minutes, upside down. Then it turned, slapped its tail another few times and then raised its head while continuing to tail slap. I guess it's an actual stretching of the spine. Anyone else that can offer a clue? I just haven't seen this very often.