I've heard that some indigenous people describe a shaman as a person that can "walk in two worlds"—the land of spirits and the material world of the living. A shaman is a man or woman is who is—either from birth or following an initiation and training—one who can travel to the subtle realms, the astral planes or underworld, converse with deities, disembodied spirits, animal spirits etc., and bring that knowledge back to this world to help their community.
In that sense, a good yoga teacher is much like a shaman.
They are able to immerse themselves in their internal practice with dedication and perseverance, commit to a lifetime of study—with teachers both dead and alive , through the scriptures or with their living guru—and bring that knowledge out into the world with them and share it with their community to help others with their own healing and spiritual development.
I was reminded of this when a favourite teacher of mine, David Garrigues posted this on Facebook:
Q: What are your thoughts on a lifetime practice?
DG: If you want a lifetime practice you have to be clear that it is long term. Rather than trying to do a perfect end point, focus on the whole body experience for each action and that will be a lifetime practice. Understand that there is always a progression. You must also understand that having a practice like this means you are ½ way between being an ascetic and a householder and that you must realize what that balance means and if you are willing to live that way.
I am also reminded of Krishnamacharya's famous "qualifications for a yoga teacher" (paraphrased):
1. They must have a good teacher themselves 2. They must have a daily practice 3. They must care for their students
So, in effect, they must be able to go deeply into their own practice, guided by teachers dead and alive, and care enough for their community to want to come out of the "yogic cave" and share what they've learned with others.
Sounds like a shaman to me?
So, you need to ask yourself: is your yoga teacher a shaman or a snake oil salesman?