[@sambauers Participant 17 years, 3 months ago]

Repost from the dev mailing list:

As of the last commit at this time (revision 1076), trunk is in an experimental and unreliable state. We are attempting to abstract some functions and classes for easier sharing of code between WordPress and bbPress. This shared code is called “BackPress”, and we’ll probably be mentioning it a few times here and there in Trac and other dev discussions.