Yoga is a secret science (Gupta Vidya), which can not be explained in books. Only with oral transmission you can understand Yoga, because its knowledge is based on "realization". When you feel the truthfulness of the information the teaching starts.
Some key issues of the teaching:
An endless energy stream shaped like a double helix (Ida and Pingala) is the fundamental building constituent of your being. There are two entry and exit points (Bindu's): one ca 15 cm above the crown and one in the perineum.
The energy stream model looks like the Yin Yang symbol and is repeated in all beings all over the universe.
There are ten moral principles and ideals (Yama's and Niyama's) to help you in correcting your behavior and making it harmonious and in accordance with nature.
To practice successfully the many postures (Asana's) you have to remain in harmony (Sattva). So you are not allowed to sweat, although you stretch your body. Each time you try to hold on the posture a longer amount of time, without movement. Your body is totally restored and becomes fit.
There are eight techniques, called energy-control exercises (Pranayama) to restore the quality and the quantity of your energy between the two points. They make you temporarily healthy, supple and strong.
There are eleven techniques, energy seals (Mudra's), to connect different energy levels with each other. By reallocating your energies you always can rely on a sufficient energy level to live on, or to improve the quality of your living.
There are four different sorts of detachment (Pratyahara's) to obtain control of your sensorial and motoric organs. They also help the control of your thoughts, emotions and feelings.
There are four steps in achieving a correct concentration (Dharana) and four sorts of concentration to achieve total health and higher consciousness.
Real meditation (Dhyana) is the resulting total abstraction of any thought, except one. Only from here on physical immortality is possible and the enjoyment of many occult powers. You have full control of your thoughts at last.
There are twelve different sorts of contemplation (Samadhi) to control higher consciousness. They let you discover real joy, love and peace. Even the activity of your heart can be controlled.
There are thirty different sorts of concentration-meditation-contemplation (Samyama's), which are the highest techniques of Yoga. This practice shows you the reality of many hidden powers you have and leads you to Self-realization, Enlightenment and Liberation.
The different techniques become spontaneous after many years of training. The way to a never ending happiness and a new magical world is open for you. You only have to pronounce the wish and it will happen.