Good question! I think it is only the exposed portion and the glans but, I really don't know that. I look forward to reading what the experts tell you.
Pressure affects everything but certainly the areas you see expanding will be the most critically hit. I am actually sitting in my car now waiting for a meeting, had the clamp on for 20 minutes and was doing several sets if squeezes noting where the most pressure seems to be. Can't attach pics via iPhone yet otherwise I'd illustrate.