Bigendersum is a gender in the bigender system in which one has two distinct genders that they are fluid between. One can be one gender or both, but never the other or a mix. Both genders, even when the individual is both, are separate. The one gender is constant while the other gender is added and subtracted from it.
The flag was created by the coiner in the coining post, on the same date. The blue represents the one of the genders that one is fluid between. The larger stripe that it is represents it is the only gender you can feel separately from the other. The black stripe in the middle represents the fact the two genders are distinct and never mix.