To understand flow, you have to read this book.
Flow is a method of behaving, characterized by little to no conscious calculations.
Csikszentmihalyi describes the experience of flow, but Csikszentmihalyi doesn't understand how flow is produced in the unconscious mind. The science in this book is a theory of flow, based on cognitive psychology experiments. But don't take my word for it. Look at the outline. Read a few sample chapters.