Whatever it is, it seems to be connected to some kind of "active" emotion. Kitashiro's completely passive in her affection for Saijou and has no drive to take action and she doesn't sparkle. Shinonome started sparkling when she explicitly recognized that she was in love with him and she's been super open and straightforward about it ever since. Yadogiri sparkles only when other people are jealous, which is like when her kink is triggered.