Rear Brake Proportioning Valve Theory

I was looking at the load sensing proportioning valve on my 1998 Grand
Voyager with anti-lock brakes, wondering how it worked. I expected it
to have some way of measuring the axle height, and therefore the load
on the rear wheels, but it doesn't. It's just attached to the frame. My
Haynes has no info on how it works. Does it sense the angle of the
body? That's pretty subtle.
-Paul