The ballerinas arms is an excellent exercise to strengthen the back, elongate the spine and reduce Lordosis (the in curve of the lower back), Kyphosis (hunched and rolled in shoulders).
Start sitting cross-legged and take one leg (flexed) to stretch diagonally in a 45 degree position. Keep the other foot slightly bent. Imagine your spine is supported against a wall. Take those arms and raise them above your head with palms now facing the top of your head and elbows looking to the side.
Twist your torso towards the bent leg. Keep the elbows open and looking to the sides.
Bend your torso towards the extended leg. If your elbow does not touch the leg don't worry about it. Flexibility will improve.
Raise the torso to its twist position again and then come back to original position.
Repeat twice to each side.
Do not allow the opposite hip to lift off the mat when in the side bend. Always keep a strong rooted position. Elbows should always be slightly bent to protect the shoulder in its socket. If you feel uncomfortable in this sitting position, just prop yourself onto a soft pillow to alleviate any discomfort.