Bigenderadd is a gender in the bigender system in which one has two distinct genders that they are fluid between. One can be either gender or both, but never a mix. Both genders, even when the individual is both, are separate. This gender can change abruptly or may have to go through a sequence of adding one gender and subtracting the other, never going quickly from one gender to the other without an in between.
The flag was created by the coiner in the coining post, on the same date. The blue and red represent the different genders that one is fluid between. The black stripe in the middle represents the fact the two genders are distinct and never mix.