(Click on the photo for a larger image.)

Notice the squatting down motion.  This squat motion is what causes the muscles in the thigh to stretch, like a rubber band.  Then in the next photo the elastic rubber-band springs back, launching her up into the air.

Her head is also straight in line with her spine.  But her eyes should now be looking up at the goal of the backstroke flags instead of down at the water at this time.  The arms should also come up quicker at this point to lead the way up and out.