Peace

 Judgement of distance

Percy is correct when he recounts Sifu's obsession with centerline. There were so many: body centerline, center of gravity, knee in relation to the hip, hand to shoulders, hand to body, knee to foot, and waist to knee, to name only a few.

The principle served as a means to define constraints. The knee should not go over the toe, for example; only in very specific cases was its limitation violated. Similarly, the outstretched hand did not carry the body's knee beyond the toe. Restrictions such as these applied to every part of the body and were one of the many things a student was to "listen" for. The accuracy of the Taiji movements depended on it.

The principal called into question the student's self-restraint. Were they able to respect the boundaries he defined, and better yet, could they find those he had not specified? It was a matter of testing for talent. Again a the process that allowed students to weed themselves out from reaching a high level of proficiency.