[There are some things you can only learn by closely watching other people.
After the last time Tobio asked Oikawa to teach him how to do the jump serve he had resigned himself to watch Oikawa's every movement to try and pick up the move that way instead of aproaching him again. Even if he hadn't felt particularly thrown off by the threat of physical harm, Iwaizumi had told him it would be best to keep his distance. And listening to Iwaizumi could never do anyone any harm.
After practice, when the gym is empty and he's the only one left, he recalls Oikawa's legs and the way his muscles moved. Recalls the arch of his body as he jumps. Recalls the way his hand moves to the perfect spot to spike the ball across the net and back onto the court before his opponent even realizes what happened.
He works until his own jump serve is a crude aproximation and then goes on to finetune it for hours every single day, and for a while it works.
But one night he can't sleep, alone in the dark or his bedroom, and recalls Oikawa's legs and imagines how they would look wrapped around him. Recalls the arch of his body and wonders what it would feel like pressed alongside his own. Recalls the way his hand moves and shivers at the thought of it sliding over his body and replacing his own hand on his dick.
He gets into the habit of masturbating to thoughts and memories of his captain to wind down at the end of the day, and for a while it works.
More and more often he finds observation and imagination overlapping, sitting on the sidelines as Oikawa practices his jump serves Tobio has to draw his knees up to hide his hard-on from the rest of the team while he bites his lip to focus himself back on the game. Once or twice he even has to excuse himself from practice to fix his problem in the bathroom because he can't make it go away through sheer willpower. And that's when it becomes a problem.
He could deal with it if it didn't interfere with volleyball but it does and refuses to miss practice over something so stupid. A sensible person would find a solution on their own, or else talk to someone who can be trusted.
No one has ever accused Tobio of being sensible.
The only way he knows how to solve a problem is with blunt force and head on. Which brings him to approach Oikawa after practice.
[If he can figure out how to phrase his problem that is.]