At some point before the fall of Wall Maria, Mr. Leonhart taught Annie fighting techniques that involved kicks and the use of the opponent's weight against themselves.
While Annie is trying to escape from Stohess District transformed in her Titan form, she has memories about her father. They are in front of a cabin, surrounded by trees, and several punching bags can be seen. Her father apologizes to her, saying that it is too late to ask for forgiveness. However Mr. Leonhart tells her something: even if the whole world hates her and treats her like an enemy, he will always be by her side. He then tells her one last thing: to promise him that she will come back.
This memory triggers her hardening ability, encasing her into a crystal cocoon and allowing her to avoid interrogation, although she is captured and placed underground.