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.