Yama Niyama are not just do's and don'ts. They are related to vision of Oneness. If the vision of oneness is lost then the practice of Yama and Niyama could be faulty. Without Yama and Niyama practice of Yoga is not possible. So what is Yama and Niyama?
Are Yama and Niyama steps of Yoga? No. At no stage Yama and Niyama can be dropped as they are angas - indivisible parts of Yoga.
