A brush drags paint. A knife lays it down and leaves. That difference is most of what separates an impasto surface from a thick painting.
When a loaded brush crosses a passage that is still wet, it picks up whatever was underneath and carries it along. Two or three passes and a clean chrome yellow has quietly become an olive. A knife does not do this: it presses the paint on and lifts away, so the colour that left the palette is the colour that arrives on the linen.
The cost is that a knife stroke cannot be adjusted. It is placed correctly or it is scraped off and remade. That is a good constraint. It moves all the thinking to the palette, where mixing is cheap, and away from the canvas, where it is expensive.