What flexibility training was BJ doing because Im pretty sure most of it is natural ability.
You can stretch all you want, only certain types of people will be able to put their own foot behind their head, especially not using hand assistance.
But one thing is certain. If you are not trying to reach your flexibility limits often, you will never know what your limits are. BJ obviously tests the limits of his own flexibility often in training.
Same with head kicks, often dudes shy away from them because they lack a lot of power due to not throwing enough of them in training. Once you force yourself to throw them, get past the weak stage, they will pick up steam along the way. Always practice both sides.