Many instructions are heard in yoga classes that have been spread over time without a valid foundation. “No Knees Past Toes” is one that is overused to a fault although accurate in some cases.
I could go through the poses for which it is true and the poses for which it is not and that would be promoting the very approach to yoga that is not serving you, namely letting your mind decide what is right for your body instead of trusting that your innate whole-body intelligence knows best.
So how do you know when it is a good thing or not? It is simple because your innate whole body intelligence always knows what is appropriate for you in each moment.
If you are told to keep your weight on your heels instead of your whole foot, you can be sure that the way you are guided to do the pose is not in line with your whole-body wisdom.
A typical example is Chair Pose. By not allowing your body weight to stay balanced above the whole surface of your feet, you may think that it is a good thing because your lower body is screaming and you have been told it is the way to strengthen.
You are strengthening alright but doing it this extreme way, you are also creating stiffness all over your body and you are activating your stress mode underground. Not worth it when you can build flexible strength instead by staying balanced and released above your feet!
All you will loose is the building of unnecessary tension, not a lessening of strength building. Who would not want the flexible strength of the cat?
For more info about this approach to flexible strength building which gives you the “steadiness and ease” Patanjali talks about, read my blogs, sign up for my free email seminar or sign up for a private session in person or online by private messaging me directly.
https://offthematyogablog.com/
Cecile Raynor has been teaching the Alexander Technique for over 25 years out of which came her B.I.A. Process to assist yogis enhance their practice bypassing the intellect. She is also a Thai Yoga Massage Therapist and a Reiki Practitioner. Faculty at Akasha Yoga Teacher Training, she runs a 12 months Mastermind for Yoga Teachers with a Vision, and a 90 Day Virtual Program for trainees, new teachers and committed yoga practitioners interested in using their body more efficiently on and off the mat in a way not taught in regular training courses. She is currently writing a book on her personal and professional experience to be published this year with Bliss-life Press, San Diego California.
Off The Mat Yoga 
Nice bblog post