Why do you need a teacher in the internal arts? To phrase it differently, we can say: What a good teacher can contribute to a student's practice? Nowadays we have books, videos, seminars by famous masters (free or paid), do we really need a personal coach to teach us the internal art?
Like every physical (or mind-body) discipline, for the internal arts, a good teacher can lead us faster on our path of learning. A tennis coach once told me that it is very difficult for him to correct the incorrect movements of a player whose muscles have ingrained with inappropriate conditioning, more so for tai chi, chi kung and other internal arts. More so because the internal arts are difficult to teach; another side of the coin is: there are not many good teachers who know how to teach effectively!
A feature of the internal arts like tai chi is, the movements are simple. A student doesn't know whether he is doing it right or not by just comparing externally his teacher's and his own movements (unlike tennis, players can see results and any hindrance to his own progress). As a result, a tai chi student may have been practising his art for a number of years but still not able to do tai chi in the correct way and therefore cannot reap its full benefit (if reaping any significant benefit at all).
What is the most important training tool of a good tai chi (or chi kung) teacher? It is the method of creating the correct chi-movement inside a student's body through hand/arm touching. It is no easy skill. Firstly it presupposes that the teacher can "experience and manage his own internal chi movement" (well, for simple movements like tai chi [or meditation!] a student really can't tell whether his teacher knows his stuff!). Secondly the teacher has to know the art of "listening to the internal chi movements of others" through hand/arm touching. Thirdly the teacher must have the experience to help "create chi movements inside his student's body" through hand/arm touching.
Needless to say in addition to this major training tool, a good teacher must also have other tools (which I shall discuss in future posts). My opinion is that this major tool is THE qualifying tool for a good teacher of the internal arts.