So what I understand is that it is white day, and Yamada noticed he had his hand in his pocket all day. Our boy was troubled because he still isnt sure if she meant to give him valentines choco, which we see she makes the effort to give him two gifts. Yamada is a clever girl, and uses the impromptu race to coax the treat from him, which to her isnt a simple snack but her white day gift that her crush has provided.
In short nee-chan got ntr'd and ichi got relationship up++