In physics, energy refers to the ability of a system to perform work. When I’m meditating, I often start noticing the sensations of a phenomenon that seems to be driving the work of breathing; I call this phenomenon the energetic aspect of breathing. (I usually start noticing these sensations when I’m approaching intermediate levels of concentration.)
These energetic sensations are distinct from the tactile sensations of breathing. The tactile sensations include the feeling of your muscles moving and the feeling of air moving against your skin, while the energetic sensations of breathing include more diffuse, wave-like sensations related to the urge (or impulse) to breathe in and out.
To sense the energetic aspect of breathing, breathe out—then wait. Eventually, I guarantee that you will notice an impulse to breathe in! Notice how the impulse to breathe in and out is distinct from the muscular movements of breathing. Try synchronizing your muscular movements to wave-like sensations of this impulse.
In my experience, attending to the energetic aspect of breathing builds this energy; as this happens, you may notice that you spontaneously start to relax and breathe more deeply. Because of this, attending to the energetic aspect of breathing is not only a mindfulness practice (that builds your attentional range), but also a vitality practice.