In Taijiquan the body is trained by certain mental metaphors and constructs that change our interaction with our physiology. Today I was teaching a Taiji class to a group of my older students who have been around a few years and we discussed the use of the bows in the body and thought I would write a bit about it just to share.
A bow 弓 is made useful by stringing the bow stave to create potential explosive energy which when an arrow is drawn is released to shoot. In order for our taiji to be done is a relaxed state of release in our techniques we have to “string the bows” or create potential kinetic energy for us to release at will. The training of the body in this way involves understanding the concepts of too straight and too round for a bow. If we take a look at the bow in upper torso which crosses the back from fingertips to fingertips we can see that we can make a bow shape very easily with a proper posture like something you would see in say Single Whip (單鞭). The arch across the back and through the shoulders and elbows creates a very powerful structure when put into good position, but we must also understand that positions need to be pressure tested in order for them to be well understood by our bodies and minds. A bow that is too straight (elbow joint too straight or the back arched) will allow the opponents power to directly affect our spine and give him the advantage taking our balance and structure. A bow that is too round (elbow bent too much) removes a great deal of the body structure and thus the muscularity we can use to do work, their power crushes through it and we are left with broken structure. The shape you see in Single Whip has the arched shape of a bow and should have the structure to handle pressure from the opponent but this in itself is still not the bow of the upper torso as it is only position and structure not potential energy.
The potential in a bow comes from stringing it by pulling the ends toward one another creating outward pressure and when we string the bow of the arms we have to have the mental construct of outward potential movement while remaining relaxed. This roundness in the upper body and arms is sometimes referred to as a ball or sphere on the chest as the surface tension of a ball is always equal and never striving, but instead is constant. Our bow in the arms must be like this reaching outwards while contained inwards creating potential like the surface tension of a sphere. This allows us to take the opponents power into our bodies and equalize it with our torso and legs while still remaining powerful and relaxed “warding them off”. The round shape of the arms makes the power break around the surface of the body so the muscularity involved is spread over the entire body and not just large or even a single muscle group. In this way we remain relaxed, powerful and full of potential. Break the strong of that bow and that outward surface tension is released all at once. Hold onto the bow while moving with an opponent and they should find no place where your structure faulters. Having power in taijiquan is not limited to relaxation but also equalization, without both sides of the coin (yin/yang 陰陽) we are not achieving taiji at all.
My two cents.