A nice way to stretch the lower back without requiring loose hamstrings
If the legs are straighter and the feet are farther away from the groin, the hamstrings will get more of a stretch. If the feet are in closer to the groin, the adductor muscles get stretched more
Good for the kidneys and prostate gland; highly recommended for people suffering from urinary problems 
Removes heaviness in the testicles and regulates periods, helps ovaries function properly, and makes childbirth easier 
Can aggravate sciatica. If you have sciatica, elevate the hips by sitting on a cushion, until the knees are below the hips, or avoid this pose entirely. Beware of hips rotating backward while seated; we want them to rotate forward
If you have any lower back disorders which do not allow flexion of the spine, then do not allow the spine to round: keep the back as straight as you can or do the reclining version
Avoid dropping the head down if the neck has suffered whiplash or has reverse curvature
Getting Into the Pose:
From a seated position, bring the soles of your feet together and then slide them away from you. Allowing your back to round, fold forward, lightly resting your hands on your feet or on the floor in front of you. Your head should hang down toward your heels
Alternatives & Options:
Elevate the hips with a bolster or cushion
If the neck is too stressed, support the head in the hands, resting elbows on thighs or a block
You could rest your chest on a bolster positioned across the thighs
Various hand/arm positions are possible: hold feet, hands on floor in front of student, or arms relaxed behind the body
If the back doesn’t like this pose, you can do the reclining variation; lie down, keeping legs in butterfly
Coming Out of the Pose:
Use your hands to push the floor away and slowly roll up. Remember, slowly means "not fast." This is a test! If you pop right up quickly, you are a yangster. Be a yinster and slowly come up
Before straightening your legs, lean back on your hands to release the hips. Then slowly straighten each leg.
Sitting up, or a gentle sitting back bend
Lying on stomach, which is also a gentle back bend
Could do a spinal lift flow on the back or flow into Tabletop (aka Hammock)
A seated twist.
Meridians & Organs Affected:
The Gall Bladder lines on the outside of the legs as well as the Urinary Bladder lines running along the spine in the lower back
If the feet are in close to the groin and a stretch is felt in the inner thighs, the Kidney and Liver lines are being stimulated.
Hips and lower spine
Recommended Hold Times:
Three to five minutes
Can hold much longer, if desired … or you may do this while reading, or talking on phone ...
Similar Yang Asanas:
Baddha Konasana, but without the emphasis on a straight spine or the feet in tight to groin. In the Butterfly we want the back to round, allowing the head to drop to the heels.
Can be done after meals, as long as head does not touch the floor (which would place too much pressure in the abdomen)
If the feet are closer in, tight adductors or lower back tightness may prevent student from folding forward. Move the feet farther away
Many students will automatically go into a tight butterfly, because of their yang training ... they should be encouraged to move the feet away, forming a diamond shape with the legs
This pose is nice for pregnant women because the legs are abducted, providing space for the belly
1 -- Iyengar, Light on Yoga.
2 -- These are the same as the ida and pingala nadis.