I have a VERY elementary understanding of this, but and I am hoping others will fill in the blanks and politely correct me if I make a mistake here which is quite likely.
So from videos I have seen there is a long vertical tube that the the say pot calls after being spun on a lathe will go into, (I have only ever seen it done with a round tube, that is why I assume it has already been worked on to some point on a lathe) a liquid of some sort is added that will harden the wood by being forced into the wood via pressure. I view it as a kind of a pressure cooker, kind of.
Anyway in some cases the maker may also add some die or color to change or accentuate the color of the wood. This is as I understand it forced into the wood, thus making it much more dense and less porous thus making it waterproof.
Again There are those that can answer better than I can, I just thought I would give a very basic idea as I understand it, others will come along and tell you far better than I can.