Ardha (means half)Pashchimottanasana is not an independent Asana Exercise). But since Paschimottanasana (full) is difficult to practice, Pashchimottanasana (ardha) is designed to enable the body to take the full position later.
Pre position or First Position: Sitting Position.
How to do the exercise:
Exhale completely, and bend downwards and touch the forehead to the knee. Don’t allow the knee to raise. Continue smooth breathing.(as in the picture)
Asana Position : The process of downward bending is the same as in Pashchimottasana (full). In this Asana one leg is folded and the calf and the thigh are kept to the floor.
Releasing Exhale, and inhaling, start raising the head.
Restore both the hands to their place.
Straighten the left leg and take the sitting position.
Note: Practice this Asana by folding the right leg making the relevant changes.
Duration Initially, even if it is maintained for thirty seconds, one experiences the benefits. Of course, after practice, it should be increased to four to five minutes.
Benefits : Basically this Asana(Exercise) prepares the body for Pashchimottanasana (full).which will have several benefits is explained in full Paschimottanasana.
Precaution : While bending downward, there is the possibility of the knees and thighs being raised, and this should be avoided.
Reference Book:- Yoga Pravesh