KenK, I do hope you mean your thumb NAIL
!
No, I mean the fleshy part of my thumb. That's just how I've always done it. I don't push hard at all. Being that its my thumb, I can feel the feeling of the blade catching on the friction ridges of my fingerprint. I can really distinguish between slightly different sharpnesses.
Think of it like you hold the blade while shaving the arm, but its just on my thumb, which provides good feedback.