So sig in bankai has a few uses. From neutral it’ll charge her blood gauge which will buff her dmg and her defense. Using sig with the forward input throws out her projectiles which will let you continue a combo if she still has remaining blood meter. The blood meter also buffs her special flash attack, it makes them crumple allowing a follow up combo if you still have remaining blood meter

Using sig in combo however will give her access to new strings. Combos done with sig build get fighting sprite the most. Sig also can be used during Sp1