Sustaining stillness in postures requires strength, concentration, and will. During an extended period of holding, proper alignment becomes a necessity. After all, stacking your bones correctly affords a lot of extra strength. It also safeguards you from injury, trains the muscles, and teaches you how to breathe through uncomfortable and difficult positions. I have found that stillness and strength, just like balance and focus, reside in the same space.
Vinyasa or flow is the fluid elongation of the breath to generate movement. Inhale up dog; exhale down dog; inhale right for forward; exhale left foot forward, each wave of the breath generating a distinctive motion in the body. It is fast, it is powerful and if allowed the sustained focus can become so deep that it shifts to moving meditation.
Flowing is liberating and makes you feel empowered; even brand new practitioners can whip there way through poses and feel like yogic rock-stars. But herein lies the rub; they will not be in alignment and they will struggle with the most basic aspect of Vinyasa, which is linking of the conscious breath to movement. In all actuality appropriate flow only grows from exploring stillness.
In Yoga there is really no beginner class or advanced class there is only practice and the willingness to work your own edge. Often what we perceive to be easy is the most difficult. Next time you go to class, put your ego down and give yourself the opportunity to learn alignment and explore your breath. Then from that place of awareness, take on the challenge of hurtling yourself through the poses while riding the wave of your conscious breath.