I believe the phrase " Swimming in Air " to describe T'ai Chi is attrubted to Ch'eng Man Ch'ing.
When I first heard it, I though little of it. I could understand it intellectually but my attention was elsewhere. Probably the exquisite pain in the legs as I tried to remain single weighted and sunk.
But a few years ago I had a different insight. When my eyes are open, I can see where I begin physically as I can see my skin. Close your eyes and that boundary is no longer so clear.
Instead of looking at "me" filling up space and swimming against the imagined resistance of air, it's the other way around. I can be defined by the space around me that creates my shapes and supports/nourishes me.
So when I practise, I focus on the space around me. Suddenly I am "swimming in air". I am released to connect.