As the body is purified through the practices of asana, pranayama and dhyana (meditation), the channels which carry prana in the body become increasingly pure and also wider. They become able to carry a greater flow of energy through the body. As this happens, there can initially be many symptoms such as a feeling of heat in the body, or tingling sensations on the surface of the skin, or rushes of energy through the body. Gradually, as the purification process deepens, we begin to feel the flow of prana as ecstasy in the body. To begin with these feelings of ecstasy can be quite localized, felt as an ecstatic connection between two places, or radiating out, in an ecstatic way from one place. Over time, this feeling of ecstasy will deepen and encompass a wider area, until eventually it will fill the whole body.
Even then it will not be present all of the time, or be experienced in the same way all of the time. There will be times when it is experienced as a powerful rush of energy flowing up through the body, or downwards from above the head. These are sometimes referred to as "kundalini rushes". At other times the ecstasy will be experienced as a glow throughout the body, or even radiating into the air surrounding the body. It feels as if every cell is vibrating with joy. At times it will be quite gentle and at other times overwhelming. However it is experienced, it is simply a product of the subtle body being purified by our spiritual practices.
As this ecstasy increases and as the bliss that we experience in meditation deepens, these two things begin to merge, or unite. This union is sometimes referred to as the union of "Shakti" and "Siva", the union of energy and pure consciousness. This merging takes place primarily in the awakened crown and in the heart. But it also happens throughout the whole body. As the merging of ecstasy and bliss become more and more refined, the crown dissolves and the heart begins to become full. As the union becomes stronger, ecstasy is felt more and more in the heart, as the ecstasy of divine love. This is the goal of yoga.