The Frog is like the Butterfly, but on your back. The principle is the same; to open your hips without extreme stretching or pain.
Lie on your back in Wuji Posture, extending your arms to the sides. Raise the legs so the feet are about where your knees were. Push the soles of the feet together and let your knees open up like butterfly wings. Look at the ceiling and old for one to three minutes before straightening the legs.