Samasthiti (Equal Standing) is a command to attention, to stand in balanced stillness. It is the practice of standing with equal, steady, and still attention. Tadasana (Mountain Pose) is the posture that invokes Samasthiti. These poses are not different. They are the same.
In Iyengar Yoga, the alignment principles of Tadasana are to be taught and reviewed before teaching almost any other posture. This helps us see how Tadasana lives in every pose. Fine-tuning and refining our understanding of Tadasana will lead to a healthy yoga practice.
In the Ashtanga method, alignment usually is not emphasized as much. We use the time in Samasthiti to come to attention and bring our awareness to our breath, coming back to our center.
This is not to say that one method is better than the other. My advice to you is to teach Tadasana in vinyasa-based classes. It is extremely helpful and necessary for students to understand and appreciate this pose. It is from understanding the correct alignment in Tadasana that the command of Samasthiti will unfold.
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.