An experiment: sit or stand erect, and let your arms hang by your sides. Lift them upwards swiftly, so that they are straight overhead. Do this a couple of times and notice how your shoulders and trapezius engage with tension to create this motion.
Now let your arms hang by your sides, completely relaxed. Let your shoulders and arms become lighter and lighter, until they are so light that they begin to float. Let them continue to float upwards, consciously relaxing your shoulders, moving as slowly and steadily as they can, barely overcoming the pull of gravity, until they are straight overhead, or as high as they can go without creating tension in your shoulders.
If you can feel a difference between these two types of movement, then you are experiencing the contrast between volitional and non-volitional movement. Volitional movement is how most of us move most of the time, by creating tension through exertion. Non-volitional movement is willed but not forced, intended but not demanded.
Your tai chi practice should cultivate non-volitional movement and avoid volitional movement as much as possible.