The point of the metaphor is you don't do your thinking in one shot. Usually you build on what you did last time and it requires effort focused at a point each

The point of the metaphor is you don't do your thinking in one shot. Usually you build on what you did last time and it requires effort focused at a point each time and a repetition of the process over a sustained amount of time. After you've thought through something, usually it stays with you. So there is some permanence to the results of the the process, much like the hole you dig. The flow thing is related to being a little lazy and listening to every whim and fancy of your mind while you're thinking on a topic. If I were to choose, I would choose depth over flow -- it's more difficult to go deep.