Yoga is unlike any other system of exercise.  The movements are done slowly.  There is constant self-observation.  If we do not have constant self-observation then we are not doing hatha properly.  It is not absolutely necessary to have only slow movements but to have full observation in fast movements too.

The muscles, the skeleton, the nerves, the breath and the mind are all coordinated and function together at the same time.  Unless a student of hatha bears those five things in mind during a given exercise, they will not be able to do the exercise correctly.  When you are practicing a hatha yoga position, practice it first in your mind. The more you relax, the easier your visualization is: you can perfect a posture without ever moving the body.  From your standpoint it would seem as if we were thinking of the body and then transferring that thought to the mind in a mental state.  But actually, where the functioning of human consciousness is concerned, consciousness comes first, the body comes second.  If you had no consciousness, what would you be?  A corpse.

So it is the mind that trains the body. (Another way it is done) in hatha yoga classes is training the mind by first training the body.  When a person actually places their body in a certain position, what they are doing is making the mind experience that position.  The mind that is in the body experiences that shape in thought. If we place our arm in a particular position, we are doing two things.  One we are giving it that shape and, two, we are having our mind experience that geometrical formation.

Do the same exercises done purely mentally give the same benefits as doing them physically?  In the long run it could but there are some very basic physiological benefits that cannot wait.  (Mental and physical working together are the most powerful.)  Whenever a student has difficulty mastering a posture it is well for them to go over the posture in the mind, over and over.

Hatha yoga builds a bridge from the body to the deepest recesses of the mind which deal with geometry, with form, with memory and which send forth the commands to control the entire autonomic system.  That is only one step.

Tense the forehead. Remain aware of the breath and how the mind is aware of the breath.  Know that “I am aware of the breath” and that “I am aware of the tension that is taking place in the muscles”.  Try to see which of the finer muscles and tissues that you are not normally aware of, you now become aware of, and try to isolate them in your mind.  Release. Let it be a lesson in self anatomy.  Let this be an exercise in thinking, not in doing.  How is the mind thinking this movement?  Observe each movement.