Ok, we all know there are a many possibilities, the 1st one it being the pose she uses to kick him in the ass, the 2nd one the she used to do karate and somehow she stopped competing and she is actually pretty embarrased about it, hence the reason of why she was mad at him when her sister said he wanted to know every little detail about her, but my favorite is that she is referencing the kick she did on the first chapter and that led to him actually praising her for it in later chapters, saying it was cool and making her blush. But then again, the main thing is that she likes what he does and he likes what she does. He thinks that the fighter nagatoro is cool and that makes her blush, knowing he likes her because of what she is capable of, seems wholesome for me