What is flow in jazz performance?

Mihaly Csikszentmihalyi has researched and written extensively on the idea of a flow state. It is a state of no effort where although a person is a causal agent the process of activity production took place with an effortless grace. The activity of being an agent during that activity goes without requiring thoughtfulness. Presumably, unconscious embodied cognitive processes have taken charge of the activity so the ego and conscious agency basically need contribute little or nothing to the process and can merely observe without seeming to actively participate in the sense of having to figure something out. The music just happens.

How and why is this possible?