Virabhadrasana I (Warrior 1) is a standing pose that teaches the actions for back extensions.
Virabhadra was a super being created in wrath by the god Siva. Described in the Mahabharata as "Mahadeva created from his mouth a terrible Being whose very sight could make one's hair stand on its end. The blazing flames that emanated from his body rendered him exceedingly awful to behold. His arms were many in number and in each was a weapon that struck the beholder with fear."
Start by using a wall for support. Walk your right leg forward and bend it to 90 degrees. Walk your left leg back and point the toes towards the wall with the heel off the floor.
Bend the back leg and tilt your pelvis to elongate the lower back. Then straighten the back leg without gripping the lower back. You should feel a strong stretch on the front of the thigh.
Now, try it in the middle of the room with the back heel down and the arms to the sides. Also called Vimasana (airplane pose)
Finally, try it with the arms up in the classic position.