A deep hip opener and one that can use arm strength, rather than letting gravity do the work.
If you do pull with the arms, the arm flexion strengthens the biceps
Releases and decompresses the sacroiliac (SI) joints
Can be a compression of stomach organs
This can become a mild inversion: a student may want to avoid this posture if she is in her
moon cycle, or if she has very high blood pressure.
Getting Into the Pose:
Lying on your back, hug the knees to your chest. Grab the soles of the feet, the ankles, or the back of the legs. Open the feet apart so that they are above your knees, and pull the knees towards the floor alongside your chest. Relax your head and shoulders down to the floor.
Alternatives & Options:
Half Happy Baby (like an upside down Baby Dragon), holding one foot at a time
If you're very tight, you may use a belt to hold your feet, or you may do this against a wall. It is like a lying down Squat, but with the feet pushing into the wall.
Can hold the back of the thighs;
Can keep your toes together for a first stage, leaving them near the groin; for a later stage, bring the toes to your nose.
Eventually, feet go behind the head! 
After a few minutes of active pulling with the arms, relax and just let the weight of the legs draw the knees down to the floor
There are two options you can try here:
Allow the tailbone to curve up in the air (releases the SI joints)
Keep the tailbone low to the ground. Notice the differences!
A deeper option that can work the hamstrings as well as the hips is to gradually straighten the legs while still pulling the feet down and wider apart, but in this option do not allow your hips to lift off the floor
Coming Out of the Pose:
Release the feet, placing them on the floor, with the knees bent. Pause for a moment.
Gentle backbends (lying on stomach) or, while on the back, a mild spinal lift, coming up only halfway
Windshield Wipers while lying down moves the hips from the external rotation of Happy Baby into an internal rotation. Lying down with your knees bent and your feet on the floor as wide apart as the mat, drop the knees from side to side.
Meridians & Organs Affected:
Stimulates the spine and thus the Urinary Bladder, and Kidney meridians while stimulation through the inner groins also works the Liver meridians.
Hips and SI/lumbar spine
Recommended Hold Times:
Two minutes if you are actively pulling with the arms but if you relax the arms, you can linger up to five minutes
Similar Yang Asanas:
Beginner's version of Yoga Nidra. Also called Window or, in Los Angeles, Dead Bug; Sarah Powers calls this Stirrup Pose
This posture is the single, most important reason that video recording equipment and
cameras are not allowed in yoga studios.