You write something clear, and readers still slide off it and forget it.
Easy writing makes for hard reading: the reader's mind holds only three or four things, so after every sentence it packs a briefcase and carries the essentials forward.
William reduced ten years of reader psychology to four things worth putting in that briefcase: ending words, topic, a sensory image, an expectation.
So is "flow" just writing to fit the reader's memory?
Largely, yes — flow is what the reader feels when each sentence hands their memory exactly what it can carry.
End on the word the next sentence will pick up, keep a consistent topic in the camera's frame, ground abstractions in a concrete image the memory can file, and open a small expectation that pulls them on.
Do that and the reader glides — and, because you packed the briefcase their way, they remember what you said.