Acts as a regulator of flagellar gene expression by modulating the protein level of the anti sigma factor FlgM upon sensing ring completion or hook elongation. Flk could inhibit FlgM secretion by acting as a braking system for the flagellar-associated type III secretion (T3S) system. Plays a role in hindering to flip the flagellar T3S specificity switch from the rod and hook-type substrates to filament-type substrates prior to hook-basal body (HBB) completion possibly by preventing interaction of FliK with FlhB. Belongs to the flk family.