Numerical and observational studies of the cosmic web show that vorticity in and around filaments is highly structured, often arranged in quadrants where the direction of rotation flips across filament branches and with halo or galaxy spins changing from aligned to perpendicular as mass or environment varies (Pichon & Bernardeau 1999; Codis et al. 2015). In ΛCDM, this intricate pattern of “spin flips” and branching vorticity requires delicate coupling between initially irrotational flows, tidal-torque theory, shell crossing, and feedback, and simulations do not yet robustly reproduce all the observed alignment transitions and large-scale coherent spins, leaving filament vorticity flips as a continuing challenge for standard structure-formation models (Laigle et al. 2015; Kraljic et al. 2020).