Delay attack was a concept that Yuki learned pretty late compared to any other tactical attack. He had never seen someone doing that for the first three or four months he was with Tsunemori High. Hell, he even survived the Inter-high competition while also facing the best school in the country without seeing a single soul doing that, which showed how rare this was for a player in the lower level.
In fact, the first time he saw it was during the test to enter the Thunder team when he and Riku had to face Tondo and Kenzo – one of the Thunder's main players – in a 2v2 game. At that time, Kenzo used a lot of tricks like that to mess up Yuki's block timing, which also opened Yuki's horizon of how a hitter should fight.
Before that, he thought that the only way a hitter could play was to jump and wait for the ball to come, and if it happened, just try to aim at the opponent's weakest spot. Not a bad insight, but that definitely just scrapped the surface of it.