It has to do with limiting any oxygen in the hot side starting with strike water all the way through boiling and pitching. The idea is that oxygen reduces malt flavors and it's specific to German styles. The goal is to get what is called the "it" flavor. If you don't know what that is, get a fresh Bitburger. It has the "it"flavor in it. It's controversial because some are totally for it and others are against saying it is unnecessary. It's also very strict in the procedure and requires a lot of equipment upgrades. I believe there is some truth to it, but I only implement some of the procedures and ideas. It can get expensive and tedious to implement completely. You can Google it, there's a lot of info out there on it.