If you ask me what is the most important training concept conducive to good progress in the internal arts, I will say it is the finding of internal resistance (in both the stationery form and the movement form). It is more obvious in the movement forms and less obvious in the stationery forms.
In the "external" arts, which include lifting weights in a gym and moving our body forward as in jogging. External resistance has to overcome in order that exercise effects can be felt and workout benefits can be obtained. In the internal arts, it is all about finding internal resistance to deliver the exercise effects. In the external form, an appropriate resistance has to be used (from light to heavy in lifting weights in a gym and from variation of speed and duration for jogging, still taking these two as examples). One major difference between external and internal: the amount of external resistance can be seen (or externally determined) while the amount of internal resistance requires a student to "experience internally" (made easier with the assistance of good teacher, but not essential for some students).
Hence, when you do your tai chi form (or tai chi chi-kung/nei-gong), if your internal sensation (and your overall workout effects) tells you that "it is so easy to do the movement", most likely you have not been able to find the appropriate amount of internal resistance. In the movement forms, internal resistance should primarily be found in the movement of your major joints (shoulder and hip/pelvic). When you cannot find it, your teacher will tell you to "relax and open" (Song 鬆). And he is right. If he is a good teacher, he will also warn you against, the other-side-of-the-coin, collapse (relax without open, 塌). And he is right again!
In stationery form (more so in seated than in standing) the issue of finding resistance is more difficult to teach and for the student to "feel" or "learn". The criteria of success though is easy to define. For example, if you do your microcosmic circulation （小周天）, if you cannot feel a strong resistance in or around your spinal cord when you move your chi up, most likely you have not reached the state. Don't worry, practise more (preferably under the guidance of a good teacher). Likewise, when you are doing chakra opening of, say, opening your heart chakra, you cannot feel that there is a resistance working against such "trial opening", you still have a (long) way to go. Again, no worry, the process of "trial opening" is internal workout per se, provided that you are focused and alerted to finding just a tiny bit more of resistance, and trying to work against it, during each session of your practice.
How about the scenario that you completely fail to understand what I am talking about? Probably you have not passed the preliminary stage and you should spend more time in doing zhan zhuang and a few simple movement forms. Zhan zhaung is far better than seated meditation for beginners - needless to say, my personal opinion only.