Takkyomi-san is right and while at that, let's not turn this situation any steamy, let's unbuttoning that blouse beforehand so next passenger can get the "message"
I'm pretty sure this is referencing the moment from Roselia's live in which they were performing ONENESS and Aiba-san (Yukina's seiyuu) does exactly that to KodouHaru-san (Sayo's seiyuu). And how Aiba-san is just smiling through it all is absolutely adorable!!