Tai Chi is moving Mindfulness. Mindfulness is a state of mind and body, aware but relaxed.
The 3 principles of the exercise;
1 Control your body by holding it upright as you comfortably can (if you have a history of back concern please see your medical professional before trying this exercise).
Allow your body to relax and start to become aware of any tension that you may be holding. Try to feel as much as possible what your body feels like while you are doing the exercise. By focusing on the exercise exclusively you are in fact doing mindfulness in its simplest form.
2 Your Thoughts -start to become aware of your thoughts and if your mind starts to wander in this exercise gently pull them back on to the exercise.
3 Breathing - Become aware of your breathing, allow it to gently move in and out, don't try to control it, allow to move in and out of your body.
The Problem with Seated Mindfulness
Mindfulness seated is a great exercise in mental development. The Buddha taught mindfulness over two and a half thousand years ago as his first meditation. It is a great form of meditation, however it is not easy. The Buddha stated;
"The mind is a drunken money dancing hot coals"
the mind never stops moving from one thing to another not allowing you to be at peace. However with a bit of diligent practise calmness can prevail.
This particular exercise video above is exercise movement number 1 of Core Energy Qigong.