Why this trick should be IMPOSSIBLE ft. Rodney Mullen - Skateboarding Science

This was probably the best video I've seen in a long time... I love both skateboarding and physics, so it was truly amazing to see them come together like this.
But, this video left me with new questions. In tumbling, we learn to flip along the intermediate axis first before adding rotation along the other axis. Why do we learn to flip along this axis when it should be "impossible".