The approach is to use the bandwidth checker to determine available bandwidth to the client, then use that information to play a video with appropriate bit rate for that connection. The buffer is not something you want to adjust for this purpose.
And bandwidth checking is not automatic. And what you do with the results of the bandwidth checking is not automatic. You have to add the module to Application.xml, and you have to run the bandwith checking functions in the player, and you have to take the result and do something with it: play a version of the video that you have encoded to a bit rate for that connection speed.
http://community.wowza.com/t/-/31
Richard