An important subject that I keep coming back to is the power of intention (yi) in taiji. Intention is such as foundation principle
that some people refer to the ultimate internal art as yichuan.
Many people lead listless, unfocused lives because they have no intention to motivate their desires. Attaining a thing or a goal requires more than thinking about it, or wishing for it: you must actively work to achieve it through consciously guided effort.
How does that apply specifically to taiji? The applicability should be obvious, at least at a gross level: you do not hit someone unless you move your fist, you do not kick someone unless you move your foot. You may move it only slightly, but at some level effort is required to accomplish the goal.
But thinking internally, there is much more. Constant internal guidance is required. When I hear people speak of taiji as meditation in motion I worry that they are misunderstanding a core concept. Unless you are practicing spaghetti taiji that has no purpose other than to feel soooo goooood, you must constantly monitor your movement and make adjustments.
Just as your taiji movement must be smooth and continuous, so must your internal monitoring be continuous. You observe your movement not just to be sure that you observe the sequence correctly, but also because you initiate your movements from your core muscles and radiate outward from there. You must constantly relax yourself and constantly accept your surroundings from all directions.