Hi Carol
You are right; engaging muscles while in a Yin Yoga posture goes against the whole point of the practice—we want to be able to relax while in the postures. When the muscles engage, they “steal” the stress away from the deeper connective tissues (see
this article to understand why.) Plus, we are in the postures for a long time, and it is not a good idea to keep muscles engaged for 5 minutes or more! It would be better to find another way for your student to do the pose without pain and without muscular engagement.
If engaging muscles makes this pose less painful, I would suspect the external rotation of this pose is getting into her her knees rather than her hips. She may need to work on increasing external rotation at the hip more in other postures. But if she wants to keep trying Shoelace, one possibility would be to alter the positioning of the legs: is the pain in the top leg’s knee or the bottom? If the top, try a block between the thighs or slide the top foot a little more forward. If it is the bottom, try sitting on a cushion, or simply do Half-shoelace, and straighten the bottom leg: this gives a bonus stress to the hamstrings. Another good option is to fold a face cloth or even socks and tuck it behind the knee. This may help create a bit of space there and take away the pain. Another option includes playing with the distance between the feet: closer or further apart. Another option: notice if the stress in the knee changes as she folds forward—that usually makes external rotation at the hips easier. If none of those ideas work, maybe she should skip Shoelace and do other postures to get to the targeted areas.
Let us know how it goes!
Cheers
Bernie