Somehow I had never heard of this. Duh-uh!
What I find very interesting is that Buddhism emphasizes impermanence--the groundlessness of existence--whereas Vedanta stresses the permanence of the unchanging ground, Brahman. Immediately I wondered about the relationship between the traditions and the forms of meditation they exercise. Sure, in Zen we can begin by examining the breath as preparation for dwelling in empty awareness, but Zen tends to use the breath, the ever-changing cycle of breath, as a figurative mental anchor; Vedanta, on the other hand, returns to the stillness of the between-breath.
So I started concentrating on the space between breaths, and since I wasn't totally relaxed, my breath was still a bit elevated. The space felt uncomfortable, like I was holding my breath more than allowing a natural pause to arise between breaths. This felt like I was underwater or somehow denying myself oxygen, probably because in order to stop the next exhalation and stress the space, I would tense my diaphragm.
Later though, when my respiration had calmed, I settled into the space. Marvelous! And yes, my awareness tended to stress the stillness of the breath more than its movement.
What interests me is the marriage of these two approaches, the attention to both the dynamic and the grounded aspects of breathing, of life and existence itself. The catch is that I wonder if I really noticed the pause before I tried this experiment. I have been meditating for years and yet I don't know if I've ever paid much attention to it! Like getting the answer to a riddle, we think, "Of course that's it!", I wonder if I had been aware of the space all along. Somehow I doubt it.
My gut instinct is that in order to concentrate on the entire breathing process in Zen meditation, it is probably most helpful to isolate the space to highlight what we may have been missing. Then, after some time, we can incorporate it alongside our breath meditation.
Stillness, breath, stillness, breath, an interdependent whole. Not one, but not two either.