First sharpen your tools ("Yi") to do Taijiquan

Video of Isaac Stern in China

"Yi when doing Tai Chi is purely mental, or one might say the intention to project"  Stephen Hwa. 
Jim R. said: You must do Tai Chi as naturally as you breathe. Or, as Isaac Stern told a young violinist:  "You must play as naturally as you sing".  I used Google Translate  for Lao Tzu's quote "..he that would perfect his work must first sharpen its tools.." This points out the import of "YI" (martial intent) and "Jing" (Force or as Master Hwa says "non-ordinary" force as found in his treatise on Fajin part 1 on Youtube).

Stephen Hwa:  "Most people are now learning Tai Chi probably for reasons other than for serious martial art applications. So the question is how much should we emphasize the martial art aspects of Tai Chi? Certainly, Tai Chi Form movements make more sense and are easier to remember if they are corroborated with the martial art origin of the movements - why there are these sequences of movements, and what the positioning of the body and timing of the movements mean. Besides this aspect, there is a more subtle but powerful reason why the martial art aspect of Tai Chi is important. I shall discuss that below: Tai Chi Form should be practiced with “Yi” (martial art intention). “Yi” is not something complex and elaborate. It is single-minded and somewhat intuitive with the desire to deliver the internal power externally through hands, arm, and foot, whatever the movement is. If the hand is moving forward, then the Yi goes to the palm and fingers; if the hand is moving laterally in a blocking movement, then the Yi goes to the leading edge on the side of the hand; etc. Once the practitioner masters the “Yi”, it is no longer a conscious effort anymore. It becomes subconscious and comes naturally whenever the practitioner makes a move. At this stage, when you play the Form, you have both the internal energy and Qi (nerve signal - a simple-minded definition) circulating in the torso of your body. With the “Yi” as a catalyst, the Qi is able to flow to your hands and fingers while the internal energy continues to circulate in your torso until you need it for an application. Then the internal energy will follow the Qi to the arms, hands, and fingers for delivery. External martial arts such as Karate also practice a solo form called Kata. Kata allows the practitioner to study movements at full power and speed and allows the student to move with the enemy in mind. So, both the Tai Chi form and Kata are practiced with “Yi”. But, Kata is practiced with a tensed up arm moving with power and speed: while Tai Chi is practiced with a relaxed arm and moving at a slow speed. The “Yi” in Tai Chi is therefore purely mental.