Anaerobic (oxygen-free) fermentation is a new method where the coffee is processed in a fully sealed and oxygen deprived fermentation tank.
First, the cherries are de-pulped and separated from their mucilage. Then, the seeds are added to a ferment tank and covered with their own tightly-packed mucilage gel. After around 18-24 hours, the anaerobic process has caused a breakdown in the mucilage and a huge buildup of CO2 pressure in the tank. This pressure forces the flavours of the juicy mucilage into the coffee parchment. Once carefully removed from the tank, the coffee is dried to ensuring a halting of the fermentation stage.
This experimental process yields unexpected and complex flavours, while also giving the producer great control over the sugars, temperature, pressure, pH and length of the ferment.
Because this is a relatively new process, there isn't a lot of information out there. Farmers experimenting with this process have worked hard to get it right, but they often don't want to share their methods. They worry that other producers will copy them, then sell their own anaerobic coffees at a lower price.