I think what he's describing is the cobra stretch.
Anyway, I'm not quite sure what you mean by 'back flexibility'. A good backbend is dependent on the flexibility of the muscles in the front of your torso - they're the ones being stretched. Good flexibility in the muscles of the back gives a good frontbend.
Anyway some stretches for backbending:
Back bridge. A good bridge should look like this http://www.gsis.edu.hk/aactivities/Images/gym_bridge.JPG
. Note how the legs are nearly straight.Cobra stretch
. Preferably put your hands on a chair or something for elevationBackhold.
After backbending, do this counterstretch
to prevent compression or tension in the lower back.
For frontbending, try to get your legs behind your head. Or stretch down like you're going to touch your toes, and pull your head through between your legs.