When your arms are very light, it’s like you had no arms. Then you can turn.Chen Yingjun

The most basic instructions in Taijiquan (Tai Chi) are given just as one starts Zhanzhuang. They are, “keep your body straight, relax your whole body”.

A misinterpretation of the second instruction above may lead the Taijiquan (Tai Chi) student to move his arms heavily. On the other hand, the desire to get stronger might lead him to execute the movements with brute force or to exert it during Fajin. These are all grave mistakes that keep the body from moving freely.
Some commonplace perceptions, like that Taijiquan (Tai Chi) movements should be heavy, that relaxation brings heaviness, and that during Tuishou the arms should rest their weight on the practice partner’s arms, are wrong, and hinder correct body mechanics. They are in fact expressions of the Double Weight error.

The Taijiquan (Tai Chi) student should feel his arms very light when practicing the form, as if they had no weight. This means the arms should rest their weight on the trunk, and not hang from the shoulders. The feeling of having weightless arms is due to using whole body power to move the arms, as opposed to using only the shoulder’s muscles, which would create undue tension.
The weight should always be on the student’s feet, which results from correct posture. There shouldn’t be blockages or deviations that load the joints with weight (actually, force). What should get to the arms and hands is Qi, not weight.