Flow is Opposition
When we think of flow we think of water flowing along like a river. Asana flow is most often described as moving gracefully, within and between poses. It is counter intuitive to think of flow as opposition, but it is balanced, opposing energy that creates flow. Flow, even of water or liquid, is only possible with the opposition of firm structures to flow over and against while taking shape adjusting to the terrain and obstacles. To create graceful, fluid movement you must have an oppositional force. This opposing force is created by isometric leverage or isotonic resistance of muscles, and less obviously, by movement against the oppositional field of gravity. A classic example is that astronauts in zero g must create resistance to work against or their bones and muscles atrophy. We are always moving within and against gravity, even while sitting still. As said in the Gita, there is movement in stillness and stillness in movement. Stillness is part of flow. Holding asanas for periods of time is part of flow, just as ponds or lakes can be part of rivers. The graceful, poised movements of yoga or dance require a balance of control and surrender, and of flow and opposition, movement and stillness. This is one of the greatest lessons, on and off the mat.