What is charm?
No, it is a saying that means the weapon you use against your target is dictated by the target itself (based on your targets vulnerabilities) And the weapon you use dictates how you go in for the attack. (An example in warfare would be using a Sniper rifle at a distance to take out an individual, or a missile to take out a larger, fortified enemy, etc).
It's a turn of phrase used in warfare. However it can be extrapolated into the realm of manipulation.
i.e. If you know your target (a woman) loves reading books at a specific book store, then you may use that as a weapon to your advantage as an opening for further action. If you uses that book (weapon), then the way you approach the target (movement) is dictated by the book itself.
So to put it all together, if you know a woman loves a specific book, you arrive at a bookstore where you know she'll be reading(movement) the book she loves(weapon). Reading a book in a bookstore is justified and expected. Reading her book in a book store adds perceived value (good taste) and a alludes to the possibility of a connection between the two of you.