It isn't an arguement. It's what Paul taught in his photoreading class. Dipping is when you slow down for a regular read. You don't stay at it long but you 'do read' a sentence or two o perhaps the whole paragraph. The thing is not to get stuck doing that and get going again with superreading asap.
So for someone who reads at 200 to 400 words per minute they are going to dip at 250 to 500 words per minute, true. Like driving a car on the freeway. You often don't slow down as much as you should when you come off the freeway. However it just becomes your natural reading speed in the long run. 
Alex
