The spoiling stance. An army that intends to win by breaking up what its opponent is trying to do rather than by out-scoring it directly.
Your Force Disposition comes out of the detachments you take when you build the army, so it is settled before the game begins, and it fixes which five primary missions you can ever be dealt.
Disruption is the disposition on 3.9% of recorded tournament lists (70 lists).
Death Guard reaches for it most, on 17.1% of its lists, then Tyranids at 12.1%.