Appear weak when you are strong, and strong when you are weak.

Not quite. Sun Tzu does tell a general to hide what he can do, but the sentence people repeat is a loose cut of chapter I, section 19, and it changes the point. The Chinese runs 故能而示之不能，用而示之不用，近而示之远，远而示之近. Giles has you seem unable when you can attack, seem inactive when you are using your forces, and seem far or near when the truth is the reverse. The passage is about feigning incapacity, inactivity and distance. That is the whole of the instruction.

The pair "weak" and "strong" is not in the section. Nor is the second half of the popular line, the order to look strong when you are weak. That reversal is a modern tidy-up of a longer passage, and the tidy-up is not there. The text tells you to understate capacity, to look idle, and to lie about the gap. Quote those three shows, in Giles's words, if you want section 19. They are longer than the slogan, and they are what the chapter actually contains.