When I see them standing like that, blushing, I think they're in their third year. Kiyoshi used to be Seirin's #1 pitcher, but he was injured. The doc said he could continue to play, but in time he's definitely going to need surgery. Since Hyuuga's pitching was only second to Kiyoshi's, he took over the #1 spot and Mitobe switched to 2nd base so Kiyoshi could catch. Thus formed the battery.
They noticed their own feelings for each other during one of Hyuuga's visits with Kiyoshi while he was in the hospital. And although they were very close friends, it took them some time to recognize the extent of the other's feelings. But Kiyoshi noticed, or rather had an inkling of Hyuuga's feelings first.
In their second year they lost a bet to the notorious fujoshi/team manager, Riko, and their pay-up was to kiss each other for one minute... with tongue. Kiyoshi calmly protested, stating that going that far was unnecessary. Hyuuga was hurt by those words but played it off by saying that the sooner they got on with it, the sooner they could get on with practice. Kiyoshi gave him a disappointed--almost hurt--look which Hyuuga assumed was Kiyoshi's way of asking him why he didn't protest as well.
Kiyoshi sighed and then stepped up to Hyuuga who, despite his urging, wasn't actually ready to kiss his secret crush. Kiyoshi placed his hands on Hyuuga's shoulders and then slid them up to cradle his head and neck. He leaned over and then whispered in Hyuuga's ear, "I'm sorry. I really didn't want it to be like this." As Kiyoshi began to pull up, though he couldn't see it, Hyuuga's brow had furrowed, his eyes had narrowed, and his mouth was twisted up in anger. But before Kiyoshi could see that face he leaned back in and added, "not the first time, anyway."
Hyuuga's face relaxed only to contort again a second later, but this time it was his heartbreak to heart swell, teary-eyed face. Until he closed his eyes, that was the face Kiyoshi saw as he kissed him. It was mostly Kiyoshi's tongue acrobatics that lead them to the finish line. Hyuuga had to give every ounce of determination and will not to throw his arms around Kyioshi's neck and fall into their first kiss. They were, after all, in front of the whole team.
It was weeks before Hyuuga could look Kiyoshi in the eye. After the kiss, Kiyoshi was certain of Hyuuga's feelings, but Hyuuga had second thoughts and wondered if Kiyoshi's comment about "the first time" was a joke. This thought kept him up at night, so he decided to not think about it. Kiyoshi sensed Hyuuga's inner turmoil and wanted to put him at ease, but thought that it would probably be more distracting for Hyuuga if he knew for sure. So, for their own reasons, they never talked about the kiss or related things again. They also made sure to never lose to Riko again.
Fast forward to their third year. The team made it to the spring invitation in their 2nd year, but their season pretty much ended there, so this year was an even greater push for Koshien. Even though the doc said that Kiyoshi's down time, rehab, and switching to catcher slowed the advancement of his rotator cuff injury and he could go longer without the surgery, the early season ending gave him more time for recovery, so he had the surgery and recovered over the summer.
During his recovery and training with Hyuuga, Kiyoshi had taken to subtly flirting and, in his own way, Hyuuga was subtly responding, but they still spoke of nothing. Somewhere along the way they made an implicit agreement to confess to each other when the time was right. They just knew that their shared right time was after Koshien, win or lose. Other than what was said right before their first kiss, they had never uttered a single word about their feelings but they still managed to convey them throughout their senior year in the most discreet ways.
So what we see in hellchou's KiyoHyuu battery picture--with its white background that represents how they've tuned out everything because this is the moment that really matters--is Kiyoshi and Hyuuga facing each other on the field immediately after they won Koshien. Still, without words, they silently confess their love.