This posture is all about lengthening both sides of the body evenly, so rather than a lateral bend (from the waist), it is hip flexion.
I love this posture because so many of us do things during the day that compress the space between the ribs and the hips (slouching!). This posture is the antidote to slouching. Seated, you lean sideways and lengthen over an extended leg while attempting to twist the torso towards the ceiling. The extended leg gets a deep stretch in the hamstring and calf.
Also, the other leg is in full knee flexion and internal rotation (ardha virasana, half hero pose). This allows the quadriceps to stretch, as well as the external rotators of the hips. This posture is a wonderful antidote to what ails you, although it is very deep and requires a lot of heat. Therefore you should see this posture towards the end of class. I like to prep this with variations of half moon (ardha chandrasana), side angle (utthita parsvakonasana) and 3-legged dog.