Interesting question. I myself have only
been interested in the color and density
change I know I'll get with certain papers.
So, can't give you an educated reason and
chemists here probably have a better answer.
But selenium works by bonding with the silver
in the prints emulsion. Thus I surmise the
selenium enhances the paper tone. If cold
tone you get more cold tone. If warm tone
you get more warm tone? frankly, I've found
some warm tone effects unpredicatable, but
not with cold tone papers I've used.

Although with Ilford Gallery (when I used to
use it) there would be a consistent color
change from natural green (chlorobromide
emulsion) to neutral regardless of how long
toning was.

Regarding only the "dmax" part of your question, according to Stephen
Anchell in The Darkroom Cookbook (2nd Edition, p. 96) the act of selenium
toning a silver print converts the metallic silver in the print into silver
selenite, which "has a deeper maximum black than metallic silver."

The various color changes themselves are enumerated and described, but not
explained in depth.

Ken

From the book:

"Selenium toners, such as Dassonville T-55, convert the silver to [silver]
selenite (examples of commercial selenium toners are Berg Selenium toner and
Kodak Rapid Selenium Toner).

"Selenite is impervious to almost all environmental pollutants, with the
added benefit that, at dilutions of 1:20 or greater,* it will not effect
image tone. Not only that, but selenite has a deeper maximum black than
metallic silver, enhancing the richness of the shadows.

* "The dilution is paper dependent.  Some papers can stand a much lower
dilution, some require more."

  The color of the original image and the color of the
toned image are both dependent on the size and  structure of
the silver crystals. In general (meaning with some
qualifications), the coarser the crystals the bluer the
image color. Extremely finely devided silver (colloidal
silver) is bright yellow. In fact, the yellow filter layer
in Kodachrome is made of colloidal silver. When toned the
structure tends to remain the same as the original. So, the
Silver selenide resulting from Selenium toning will be
bluish or purple on cold tone papers and on film and more
red or yellowish where the original silver is finely
devided. Sulfiding toners are similar, the coarser the
original the colder the brown. In fact, Polysulfide toners,
like Kodak Brown Toner, result in a slight cooling of the
color of microfilm when used to protect the image. This is
because the color of Silver sulfide is also dependent on the
structure of the crystals.
  Very cold tone papers are usually toned by bleaching them
first and then treating them in a sulfide bath. The image
color still depends somewhat on the original color but not
so directly because the original silver is converted back to
a halide by the bleach before toning. Direct toners, like
KBT or Kodak Selenium toner, tend to preserve the original
size and distribution of the image silver.
  KRST and some other toners (Gold for instance) intensify
the original image. This is NOT because they are "blacker"
then the original silver but because they add to the
covering power of the silver by increasing the size of the
grains a little. Actually, silver can be used for
intensifying, for instance Kodak In-5. This intensifier
increases density without chaning image color.
  Indirect Sepia toners (bleach and redevelop type) can
also be intensifiers provided that there is absolutely no
hypo remaining in the emulsion. Any hypo will combine with
the bleach to form Farmer's reducer and will slighly lower
the density rather than increase it.