This one is covered in Jack Dykinga's book on Nature Photography, if you can find a copy, and shows examples of what you are asking.
Basically, you are changing the plane of sharp focus to allow the near-far relationship to come into focus with the lens. The same type of relationship can be accomplished with rear tilt to accentuate the foreground, where you want to have near objects loom and look "larger than life." What is happening is the small amount of tilt (you did use a very "small" amount of tilt, didn't you?) employed to change the plane of sharp focus must be made up for by stopping down. In the same way dof is used by hyper focal distance and f stop, the middle ground is brought into sharp focus by stopping down once the near-far objects are brought into focus.
A polaroid back will help give instant feedback to see this one. Once the near-far objects are brought into focus, the f stop is used to bring in the middle. Takes a bit of practice, but there are books which cover this subject (Steve Simmonds, Leslie Strobel, etc). tim