Danny pretty much nailed it. depends on the specific process, but typically some form of process whether via pressure or vacuum is used to draw a resin into the wood. I stabilize my own woods in a lot of cases. I have a vacuum chamber which I use along with a two part epoxy catalyzed resin.
Heat is also used in conjunction with the vacuum. More times than not, you are talking acrylic resins rather than "epoxy" resins...but the end results are fantastic. I have always used a man named "Nicholas", which is his business name as well. He's the absolute #1 man in the biz. I have used WSSI and have have had very poor results with their product for what they charge.
My last piece he stabilized for me was a chunk of cypress that was carbon dated at 48,750 years old +- 500 years...it turned out eye popping.
