how is consciousness define in yoga/meditation?

Answer 1

The Yoga Sutras of Patanjali defines yoga as:

yogah cittavritti nirodhah
Yoga is the cessation of movements in the consciousness.

The union we seek with the bliss-void is possible since that reality is our most real reality. Suffering is caused by our immersion in delusions that block us from our here and now, real, free condition. Therefore, the union is not some mysterious state, artificially constructed beyond everything—it is merely the real "here now" once there is cessation (nirodhah) of the habitual functions of our self-centered minds that compel us to suffer unnecessarily. Nirodhah is the third noble truth taught by the Buddha, the noble truth of the cessation of suffering. It is a synonym of nirvana, the unexcelled bliss of ultimate freedom.

It takes a particular effort for us even to imagine such a freedom. Who today, even in this "land of the free," really thinks it is possible to reach a complete experience of absolute freedom that is not just a numbing annihilation of oblivion but a positive, eternal bliss of inexhaustible satisfaction?

Who has developed the ability to imagine that the essential reality of everything here and now—every cell, every fiber, every atomic or subatomic or quantum energy—is ultimately nothing other than infinite bliss, eternal life, personally enjoyable by each of us and universally sharable among all of us, including God and all Gods, Buddha and all Buddhas, completely one with each and every one of us, without eradicating the differences that enhance our mutual bliss?

A moment's reflection lets us know that our conventional imagination of reality is very different. We are conditioned to believe that physical mechanisms completely determine our existence.

We can bear such bondage only because we are misled into thinking we can slip through the gap of ultimate randomness—the random nature of the Big Bang, genetic mutation, the evolution of life from inorganic matter—into a subjective release into an annihilated blank nothingness.

This latter is a kind of imagined freedom, an imagined superdeep sleep state of release from the burden of subjectivity and its danger of pain. But it is not possible to attain, and even if it were, it would be far from supreme liberation—ecstatic being-awareness-joy, bliss-void, nirvanic reality!

Answer 2

In general, you're tapping into your unconscious mind since you're giving it a little bit of a break by consciously controlling your breathing (which your unconscious typically does).

This site discusses it a bit; it's heart rhythm meditation practice, which I'm enjoying very much right now. :)

http://www.appliedmeditation.org/