Invisible strength, as defined in the beginning of the story, is the ability to win arguments and win respect from people. This is a broad definition, and it can be applied in many different ways. In light of this, let me offer a few examples.
First, Waverly admits that this art of invisible strength could be used to win at chess. She could lure her opponents with feigned weakness and then strike to deal a mighty blow. At one point in the story, she make this very point. In a tournament, she states:
The wind blew stronger. "Throw sand from the East to distract him." The knight came forward ready for the sacrifice. The wind hissed, louder and louder. "Blow, blow, blow. He cannot see. He is blind now. Make him lean away from the wind so he is easier to knock down."
Invisible strength is not limited to only games. It can also be applied to life. The clearest example of this can be seen in Waverly's relationship with her mother. Through the story, Waverly wants independence. Her mother allows a certain amount, but she insists that Waverly always go to the market with her. Her mother, Mrs. Jong, likes to boast about her daughter and her accomplishments. This point annoys Waverly. So, at one point, Waverly confronts her and runs away.
When she comes home, she has to face her mother. She knows that her mother has the position of strength. To put it in terms of the art of invisible strength, Mrs. Jong is winning. This is precisely why Waverly sees her relationship with her mother as a game of chess. It is contest of wits or invisible strength.
Her black men advanced across the plane, slowly marching to each successive level as a single unit. My white pieces screamed as they scurried and fell off the board one by one.
The story ends with these moves: "I closed my eyes and pondered my next move."