it's explained in section 4.2 of the paper https://www.comp.nus.edu.sg/~bleong/publications/sigmetrics2...

afaik 1st-gen bbr is somewhat unfair to other cc-algos to the point that it will starve them. use it if you want to be 'that guy' (or google :)

update: slide 27 and onwards https://www.potaroo.net/presentations/2018-05-10-bbr.pdf