While looking for some further details on this pose - which comes after Hand To Big Toe (Utthita Hasta Padangustasana) and before Pyramid Pose (Parsvottanasana)in the Ashtanga sequence - I came across this article from Yoga Journal as written by Beryl Bender Birch: Ardha Baddha Padmottanasana
In 2005 I went to a workshop conducted by David Williams – he skipped this one completely with the reasoning that most people dislike the whole balance/twisting/bending aspect so why do it standing at all when you are just going to repeat it in 10 minutes on the floor in seated variation? Interesting point.
Yet, it’s a good pose to know. If you are a cyclist, an endurance athlete, runner or triathlete, it's a great way to stretch the hip. I love doing this stretch after Spin class or a long ride.
Half Bound Lotus Standing Forward Fold
(From the Ashtanga sequence)
You’ve just finished Side B of Hand to Big Toe pose (Utthita Hasta Padangustasana) in your variation and returned to samasthti at the front of your mat.
Press your LEFT foot toward the earth.
INHALE, bring your RIGHT foot toward your left hip flexor:
NOTE: I cue “toward” your left hip flexor. Please listen to your knees and hips. If such an extreme flexion is not appropriate for you, take Tree Pose (Vrksasana), or a Standing Pigeon Variation.Ardha Baddha Padmottanasana
Hold for 5 breaths.
This is another pose where there is some deviation in what to do with the arms. One variation has the practitioner keeping the opposite hand at the hip. The other variation has the practitioner raising the arm over head and keeping it extended as they forward fold. Which variation you do will depend on when your instructor learned the pose, and who they learned it from. Either arm position is correct.
Bent knee – try and keep the bent knee pointing somewhat down toward the Earth and not jutting out toward the side. Again, how you are built will dictate where in space your knee is.
PLEASE PLEASE PLEASE! IF you have had knee surgery, knee issues, hip surgery or hip issues modify modify modify!
If forward folding, try not to hyper extend the supporting knee. Be careful exiting the pose – bend the supporting knee. Engage your core.
Do the same thing on BOTH sides. If you keep working toward your strong side, your weak/injured side will not have a chance to catch up, and you will create an imbalance in your body. Modify the same on both sides to achieve equilibrium, then move forward from there.
Alternatively, do tree pose (Vrksasana) standing, and work on the nuances of the pose when you come to it in the seated sequence. When you feel comfortable with your balance, then start to build from there.
Pictures taken from the web by Googling ardha baddha padmottansana. Video is from YouTube.