Start first to look at the Sanskrit breakdown of the word - yoga, or Sanskrit "YUJ" - the translation of which is, yoke, control and the most commonly used understanding - union. Yes, but union of what?
Looking at it from a basic view, it is the union of the 3 bodies, the physical, pranic (astral) and mental body. The control of the physical body and its messages (asana) through the use of intelligent breath (pranayama), then the mind can be at rest of the physical messages. A teaching that is stated in texts and left without any further explanation.
To understand this statement and to understand what union is, there needs to be an understanding of the final goal of yoga as stated by patanjali in the yoga sutras.
Samadhi is the fading of the meditation object and bliss, the apparent fading of dualism. The mind expands and enters into a bright awareness, equanimity. This state of Samadhi allows the mind to dissolve all ideas of separation, concept and feelings from self and object. Self/object and self/other dichotomy.
Samadhi cannot be reached in asana or pranayama but the practice of these 2 limbs along with reflection and cultivation of correct yama and niyama will result in a calmer mind and a physical and mental environment that will certainly help toward attainment of Samadhi.
Samadhi is the result of correct union of awareness with awareness itself. When the sense gates and mental fabrications have faded from attention and within serenity direct knowledge, direct experience of mind is realized and the trappings of labeling evaporate into emptiness.
In this modern day, the common practices of yoga are asana and pranayama. So what is union within this mode of practice.
The practice of asana helps to bring the muscular, skeletal and nervous systems into alignment, bringing the body into a healthier state. Asana balances out the physical limitations which results in better blood circulation and an overall healthier organism. In other words union of the workings of the physical aspects of the body. This in turn presents itself from the outside perspective as a healthy looking, strong and flexible body.
The practice of pranayama helps to control the respiratory system and minimize it limitations. Using more of the body in the breathing process and keeping the internal workings of the body oxygenated and healthy. Union of the body and breath.
Union of asana and pranayama results in control of the bodily messages, an understanding & acceptance of the sensation of the stretch, a control and release of personal limitations and a heightened sensitivity of the compounds of body and prana.
Because the mind and will leads all asana and pranayama practice, this path leads to the union of the physical body, pranic body and mental body - 3 body union.
This page is provided by Kim White of the Samadhi Centre Gold Coast
Kim White created the mind body yoga system of the runs International Yoga and internal art schools - Yoga Phuket