When does this union occur? When the mind becomes quiet. This mental stillness is created by bringing the body, mind, and senses into balance which, in turn, relaxes the nervous system. Meditation begins when we discover that our never-ending quest to possess things and our continual craving for pleasure and security can never be satisfied. When we finally realize this, our external quest turns inward, and we have shifted into the realm of meditation.
Expanded State of Awareness
When we are grounded physically and mentally, we are keenly aware of our senses, yet disengaged at the same time. Without this ability to remain detached yet observant, it is not possible to meditate. Even though you need to be able to concentrate in order to meditate, meditation is more than concentration. It ultimately evolves into an expanded state of awareness.
When we concentrate, we direct our mind toward what appears to be an object apart from ourselves. We become acquainted with this object and establish contact with it. To shift into the meditation realm, however, we need to become involved with this object; we need to communicate with it. The result of this exchange, of course, is a deep awareness that there is no difference between us (as the subject) and that which we concentrate or meditate upon (the object). This brings us to the state of samadhi, or self-realization.
Source: "A Beginner's Guide to Meditation, by Mara Carrico, Yoga Journal, (2007). http://www.yogajournal.com/article/practice-section/let-s-meditate/