Kneeling is the most primal sign of showing subservience to the strong. Cavemen in the ancient times did it. Warriors in the Classical period did it. Knights in the Middle Ages did it. Shounen kino in the modern age did it. Shitposters in 4chan post kneeling pics when they concede. Once you kneel, it is over. You lost. Simple as. The writers could have shown Hoshino lying on her back, tired from all the fighting but they specifically chose to show her kneeling to Hina instead to confirm her victory.