This is the first step to mastering (the following) full back bend. Please only do it when you have thoroughly warmed up since the muscles can be damaged severely when doing such bends with cold muscles. 

How to: Kneel down and place one hand on your foot. Lift the arm parallely to the head. Look up and breath in firmly through the nose and out through the mouth. Since this is an intermediate Yoga pose don’t overdo it and start small with 1-2 breaths and then loosening the pose again. In the next step, you might want to try it out with the other arm.

I have come across this pose in my life Yoga classes, so no link that I provide you with this time.