Bigenderside is a gender in the bigender system in which one has two distinct genders that they are fluid between. One can be one gender, both, or a mix of the two, but never the other one. This gender can change abruptly or any other way.
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 represents being distinct and separate. The purple represents a mix of the two genders. The white represents fluidity.