Throughout the series, Yamaguchi's jump float serve improves in terms of consistency, speed, and accuracy. He initially struggles with consistency and nerves but gradually gains more control over his serve, leading to more successful and strategic serves that put pressure on the opposing team. Additionally, he learns to vary the placement and spin of his serves, making them more difficult for opponents to receive effectively. Overall, Yamaguchi's jump float serve becomes a valuable asset for Karasuno's team as the series progresses.