This is yet another version of crescent moon posture.

For beginner, you may rest the back knee on the mat.  If you want a challenge, you may come up to this posture from Warrior 1.  This posture helps you to strengthen your back, arm and shoulder at the same time opening up the chest and hips thus improving balancing.  Look up to the sky instead of dropping your head to the back which may hurt your neck.

Namaste, everyone!