What is healing? The best starting point for any on-line research is Wikipedia. And there it says "Healing (literally meaning to make whole ) is the process of the restoration of health to an unbalanced, diseased or damaged organism. Healing may be physical or psychological and not without the mutual reception of these two dimensions of human health." In the arena of psychological or spiritual practice, healing oftentimes refers only to psychological healing, and depression in particular, a condition that seems to be affecting more and more modern men and women.
Many people do not like the idea of visiting a shrink for fear of their condition being labeled with psychotic illness (or worse, "getting crazy"). And some simply don't like the idea of taking psychotic drugs (which their shrink might actually be taking themselves due to their heavy workload!). Luckily there are now many alternatives for less severe cases, like music therapy, drama therapy, drawing therapy, and of course tai-chi/qigong/meditation or other mind-body exercises.
It is interesting to note that seasoned athletes are usually less prompt to have psychological ailments. In the old days, harsh physical activities had been used to "cure" mental illness and criminal behaviors (and sadly to some spiritual or political dissidents too, which still happen nowadays in some places of the world).
The common element on all these healing methods is subtle muscular control by a focused mind. Take an example drawing. In order that drawing can be used for healing, a student needs to observe his objects, say three apples, carefully. And then he shall exercise his focused mind in conjunction with his body (mainly his hand) to draw the three apples as he sees them. Without a focused mind, a student might render the three apples as the same, but with a focused mind, he will be able to observe the fine differences and will be surprised to find out that each apple has a unique "personality". His focused mind will need to stay calm because he need to train his muscles to contract (and relax) subtly in order that he can deliver such differences in his own drawing. And the result is therapeutic.
In doing tai-chi/qigong/meditation, it is the same. A student practitioner has to focus his mind on the internal subtle movements of his muscles (many of them will be quite "involuntary" in the beginning). The most important muscles will be his breathing muscles. And these will act as the core muscles to subtly activate the student practitioner's whole body muscles in turn. The internal feeling of chi is the guidance for the student. For the movement form, his teacher/coach/sifu can help to check his progress - like in drawing, checking on whether what his drawing matches with what the external reality is. And the Qigong teacher's role is to give instructions for corrections, just like a arts teacher does to his students.
The situation however gets a little bit complicated with meditation, and more with seated mediation. External clues for good performance is difficult to identify even for experienced teachers. And without such clues, a teacher might not know whether the student is progressing or not. Worse, if there is no improving subtle muscular control on seated meditation, without much external stimulation, the psychological ailment of a student will get worsen instead of better. And indeed there are stories of young people going to mediation in secluded monasteries (like in Thailand where westerners find more welcoming) and come back with psychotic onset and need psychiatric hospitalization.