Esoteric teaching is imparted in the oral tradition, from master to
disciple. The word esoteric means
that it is intended for, or likely to be understood by only a small number of people with a specialized knowledge or interest.
Authentic Tantra teaching is not available
to students in written form. Not unlike the relationship between
psychoanalyst and patient, the counselor in this care must also manage a
relationship between herself, the trainee and the particular aspect of
training appropriate for that stage of development. The Acharya relies
on feedback and his/her observations of the devotee's advances. It cannot
be done by reading a book or watching a DVD.
It takes a skilled teacher and a very good teacher-master
relationship to get it right. Esoterics are what they are because the
subject matter goes beyond what body-mind can comprehend, beyond what
language can illustrate. It has to be learned in an existential
experience and cannot be learned by reading about it or in a classroom
setting. Most people are on a path that does not require esoteric
teaching to move ahead. Some authors postulate that less than one in
every ten thousand devotees will benefit from esoteric teaching.
Is the teaching secret?

Yes, all authentic tantra traditions have a a vow of secrecy. The reason for that is not
by any means an elitist one, it is profoundly practical. To illustrate, I
will mention one example that may or may not be something you will
encounter in training...it may be fictional. Lets assume that you have
reached that level of training where the Acharya must introduce you to
the concept that you should, and can, take any physical experience (such
as pain), or emotional experience (such as rage), or chemistry
experience (such as sexual desire) and turn it into energy which you
master and channel into the higher chakras to achieve levels of
spiritual insight that you have not yet seen. The acharya knows the
devotee well and manages technique and skill level. Acharya knows that
the devotee has already mastered other techniques, without which this
new phase can be dangerous. Now, lets assume that this information is
written in a course and a devotee follows it. The reader, seeking that
elusive quick-fix pill, or holy grail, the speedway to enlightenment is
not ready for the technique and the candle actually burns a blister on
her hand, she flies into a rage and kicks over the table and sets the
cat alight--and the doctors in emerge go "
tsk tsk, yet another tantra casualty, that stuff should be banned."

One way of describing the goal of Tantra is that one learns, the fast
way (one lifetime rather than many) to master the body and body-mind
and get them to work for your spiritual development rather against it.