Let’s look at engaging the legs more strongly in this challenging pose. When the legs are slack, the pose is MUCH harder. (Check out this video in which I discuss the difficulties of relaxed legs and demonstrate another way to ensure legs are engaged than what I am discussing here.)
In the above photo, the student is engaging her upper body and her legs without lifting the hips. This way, she can play with what her legs are doing without fatiguing. Her partner is pressing on the heals with light weight and pulling gently to elongate the leg. The pressure allows the student to “feel” the back of her legs more so she can engage them more deeply.
You can recreate this yourself in plank with your feet against the wall. In this pose, the heals can press firmly, making the legs very strong. (A great way to teach plank, too!)
When the student lowers into the full pose, her heals will move away from the wall. The partner pressed her hands on the student’s heals so the student still had something to press against.