The clue is in the name: intense side stretch. The work in the legs is intense too. The front leg is working as in Uttanasana, the back leg as in downward dog pose.
I have scoliosis (curvature of the spine). Parsvottanasana is an added challenge because my pelvis drops to one side and my rib cage turns to the other.
More experienced students take the hands behind in Paschima Namaskarasana to achieve full Parsvottanasana (see photo). This helps open the chest but makes it harder to keep the hips level and take the head down.
Starting to learn this pose, you take the hands to the floor, or to bricks. This teaches the body the fundamental movements in the legs, hips and trunk before progressing to the full pose. In the last two years I’ve come across several good methods to practise the pose, from ropes, bricks, wall to chair.
I particularly like the use of a chair. Eyal Shifroni’s excellent book, A Chair for Yoga, shows two excellent methods for keeping the hips and the trunk level and for achieving a good, straighter final pose.
I’m beginning to understand Parsvottanasana and why I find it difficult, and I hope I’m beginning to improve how I do the pose. Recently I realised that I’ve even begun to like it!