The chapter on empty and full spoke of initiative over place and time. Now it moves to how you use your strength once you are there.
The core is one thing. Make them visible, and stay invisible yourself.
故形人而我無形,則我專而敵分
gu xing ren er wo wu xing, ze wo zhuan er di fen
So if you make them show their shape while you have none, you gather into one and they split into many. (The Art of War, Empty and Full, translated directly)