Our breath is called the flow of life in various traditions, using terms like prana or chi. This prana is an effect of the flow of Shakti, the feminine principle.
Before there is anything, there is 2 principles. Alertness and liveliness. When liveliness stirs alertness, it becomes conscious and begins to flow. Then it becomes conscious of itself and all experience, all creation is born.
This liveliness, this flow is Shakti. But because this liveliness and flow is in the nature of consciousness, of observation, it cannot be known as an object in the usual way we experience things. While we might say Shakti is energy, this rather underestimates it.
As others have observed, there is no sensory mechanism for experiencing Shakti itself or even kundalini. Much as you may have read experiences of “rising kundalini”, what they actually describe is the effects, not the Shakti itself.
Similarly, Kundalini Vidya describes how the crown chakra manages the opening of the petals of the other chakras. But we don’t experience the crown at work. Rather we experience the effects, the prana, vayu (air) or energy moving, particularly with openings and releases. And even there, if the process is mild we might not even be aware of it.
There is, however, a different dynamic that can take place later on. To understand this, it can be useful to look at the progression.
First, we recognize ourselves as consciousness in Self Realization.
With refined perception, we begin to notice that lively quality in everything in the environment. Perhaps we notice this as light around objects or energy or vibration. (of course, some value of this is possible well before)
After the Unity shift (typically), we recognize that lively quality is an effect of flow, that everything in the environment is flowing, is flowing awareness.
And we can recognize we are that and we become the flow. Then we can experience ourselves as both observing and being the flow within that.
When we recognize ourselves as the flow, then move more deeply into the flow, we know ourselves as Shakti. We are that which flows through everything and animates all life. We are our very breath, our very life itself.
And curiously, the in and out of the breath is recognized as a superficial value. The flow itself is circular, flowing ever forward.