"The weighing process will either employ volumetrics (submersion in water) or require [the person] to lie on a table with his head extended over the edge onto a scale."
The volumetric method would seem to assume that the density of the human body either does not vary significantly, or that the relative density of the head (versus, say, the knee) is known.
The "head over the scale" method doesn't sound accurate. How to determine exactly how much of the body you were weighing, given that the head, neck and other parts would still be attached.
So, how much does a typical human head weigh? The same site notes:
"According to the Department of Anatomy & Histology, University of Sydney, an adult human cadaver head cut off at vertebra C3, with no hair, weighs between 4.5 and 5 kg (between 9.9 and 10.23 pounds)."