In Akito the Exiled episode 2, there is a female scientist talking with a person in a lab tank and she mentioned about "BBT event". It seems it was about the four 11s being synced in battle. But still, what does it mean?
The scientist says "BRS", not "BBT" (this is around 54:12 into the episode).
Odds are that the "BR" stand for "Brain Raid" (which is a thing that is mentioned earlier by the same person, around 46:37). Japanese Wikipedia claims that the "S" stands for "System", though I don't know where it gets that from.
The official website describes "Brain Raid mode" (ブレインレイド状態) as a feature of the Alexander (the mech that Akito pilots):
When the Alexander enters Brain Raid mode, its face mask deploys. During a Brain Raid, the pilot's mind "extends", allowing it to subsume the minds of other Type-02 pilots, allowing their mechs to be controlled as extensions of the main unit.