Badger brushes hold water differently than boars, they don't absorb water at all. They trap water in between the bristles.

I hold mine under hot water for about 15 seconds to heat the hairs up. Or dunk it once or twice in hot water.

Soaking offers no benefit whatsoever.

So, here's the technical answer:

"Badgers have, what we in the fly tying community refer to as, solid hair vs. species such as deer, elk, moose, antelope and caribou which have, predominantly, hollow hairs. Structurally, solid hairs are not actually solid but rather, under the pressure of tying thread, they behave as though they were. Such hairs are incompressible and do not flare or deform as they are wrapped with thread. The surface of the fibers tends to be rather smooth, almost slippery, and when gathered in a bundle, these hairs will slide and roll against one another. My conclusion would be that badger hair hydrophilic properties are not such that it absorbs water, per se, although there is some penetration in the animal when alive because of the permeability properties of the skin which allow for follicle opening and, thus, some water retention. Absent of the skin, I believe that in a shaving brush it is the architectural structure of the instrument that accounts for water retention."

There are actually little "pits" in the surface of badger hairs that "grab" water molecules and hold them between the hairs. As the hairs move, they release the water easily. This property is what we call "flow through" the higher the loft, and the less densely packed the brush is, the better the flow through.
Here's a magnified photo of badger hair:

Here is a badger hair under an electron microscope. In this case, a Shavemac D01 Three Band hair:

The tip of that hair:

Here is a human hair under an electron microscope:


Boar hair is actually hollow, which is why it becomes waterlogged and softer and it soaks up more water.

Boar shaft under an electron microscope: