As my meditation has begun to deepen, I have begin to notice some rather new sensations. Probably around 20-25 minutes into my sitting, when I begin truly accepting my body and mind as it is, I feel a strange tension mostly in my stomach and abdominals (Its almost as if I feel the tension inside the organs themselves as well). This is not me clenching the muscles since they are relaxed, from what I can tell it seems to be more associated with anxiety that I might have built up during my day from not maintaining mindfulness. That is, the sensation seems to be the emotion of anxiety itself.
Those who meditate deeply become deeply self-aware. Often we find in this self-awareness things about our self that we are not comfortable with, such as anxiety. So, we let go of the anxiety, and meditate deeper.
So, as I accept this sensation, the tension builds until I have let go of it entirely. At this point the feeling of tension (not actually physical tension) changes entirely turning into what I can only describe as a liquid flow of energy that then goes up towards my head. At this point my mindfulness and concentration increase very suddenly. Sometimes this happens more than once during my meditation, the feelings of tension in my stomach seem to re-arise when my mindfulness lags, and dissolve in a stream of energy when my mindfulness and concentration reach a certain breaking point.
Yes, as we meditate for longer periods of time we may notice waves of movement within the meditation experience. When our focus waivers, then we lose depth. As our focus becomes better, then our meditation experience improves.
However, to use the English contemplative language better, then we would say when we are engaged in a meditation object, that is the practice of meditation. Concentration is what we are doing when we are engaged in a meditation object.
When we are having a meditation experience, such as pleasant feelings, energy, etc. then we call it absorption, or a meditation experience, or a religious experience, or jhana, or samadhi.
EDIT: Reviewing this immediately after a sitting, I realize that I actually have no idea what the strange tension is. I was assuming it to be anxiety/stress, but now I am uncertain.
The "tension" that you are experiencing could be a charism, in which you are noticing spiritual energy (virtue, virya, kundalini, prana, qi) arising in your stomach chakra.
