Read Maty Ezraty's response:
My teachers and my study of yoga have taught me that bandhas should unfold naturally from correct practice. "Bandha" means to bind your attention. It is a way to focus the mind on one point, which ultimately is your true self. The bandhas are more about the mind than the physical body.
If we teach bandhas from a purely physical perspective, many students will likely misunderstand. The consequences of practicing bandhas incorrectly are significant. Holding the anus and belly forcefully can lead to constipation and create a tucking in the lumbar area, reversing the natural lumbar curve. The misunderstanding of this practice can also cause some women to skip their menstrual cycles.
The practice of bandhas is so refined and specific that it may not be appropriate to teach in public classes. I like to have a personal relationship with a student before attempting to teach bandhas.
Most beginning students have plenty to do in simple poses, so including bandhas in your instructions might be overwhelming. Standing upright in Tadasana (Mountain Pose), or sitting in a simple cross-legged position, is very difficult for many beginners. The back rounds while sitting and the chest collapses while standing.
Learning to breathe correctly takes time and practice, and bandhas will not come naturally until the physical body is well aligned, and the breath is free and without force. By the time the student is ready for these teachings, she or he has probably discovered them for themselves through regular practice.
Therefore, until that is accomplished, I do not recommend the teaching or even the practice of bandhas. Learn to teach the postures well. When you are ready to teach bandhas, you will not need to ask a teacher how to do it. It will come from your own experience.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide. For more information, visit http://www.chuckandmaty.com.