All other details – breathing, mistakes to avoid, leg loosening and advanced practices in the final pose – are as given for janu sirshasana.
Details on awareness, sequence, duration, limitations and benefits are as given for paschimottanasana .
Massage of the abdominal organs
Though the benefits obtainable from ardha padma paschimottanasana are almost identical to janu sirshasana and paschimottanasana, it has one distinctive characteristic: the foot of the folded leg applies an intense massage to the abdominal organs. Each leg is folded in turn; this massages each side of the abdomen, which helps to stimulate intestinal peristalsis and thereby alleviate constipation. In this context, one should remember that faecal matter in the bowels first of all moves up the ascending colon on the right side of the abdomen and then down the descending colon on the left side of the body before being pushed out of the anus. Therefore, it is important that the left leg is folded first (the left foot pressing and massaging the right side of the abdomen) so that the influence of the asana is in accordance with the movement of faeces within the colon.
All the other abdominal organs are also given an intense massage; the liver mostly when the left leg is folded and the spleen mostly when the right leg is folded; the kidneys each receive an equal massage alternatively; and the pancreas receives approximately an equal massage from each foot in turn. The other organs, such as the bladder, adrenal glands, etc. are also massaged.
To obtain the best possible massage of the internal organs, the foot of the folded leg should be firmly pressed into the abdomen. If you do the advanced technique where the toes of the folded leg are held by the arm behind the back, then this will happen automatically. If you do the simplified form of this asana, then you should take care that the foot presses as firmly as possible into the abdomen before bending forwards.
This intense massage of the spine will only be obtainable if you can bend forwards sufficiently to press the foot into the abdomen.
Until this stage you must slowly loosen up the back and leg muscles and not worry too much about the benefits obtainable from the massage of the abdomen.
For fuller details of benefits refer to the topic on paschimottanasana.