WARNING: The following contains spoilers for Season 1 of Record of Ragnarok, now streaming on Netflix.
In Netflix's Record of Ragnarok, Valkyrie Brunhilde's team struggles to overcome Zeus and his pantheon of gods during the titular tournament that will decide the fate of mankind -- survival or genocide, depending on whether mankind or the deities win overall. The Valkyrie power-ups do make humanity's champions stronger but ultimately, the gods have abilities beyond comparison -- which is why Brunhilde knows it'll take something extra to win.
Luckily she's got the Japanese swordsman, Kojiro Sasaki, to fight for her in the third match after going 2-0 down, and while he's considered the world's greatest loser, he delivers an astounding win thanks to a secret technique.
Kojiro isn't the best swordsman who's ever lived in Record of Ragnarok. In fact, he's not even a legend. Members of the crowd are quite apprehensive since there are fully proven warriors there, such as Musashi. As for Kojiro, he doesn't have a single win under his belt. His record is filled with losses, so folks are mystified that Brunhilde recruits him.
But despite Musashi having killed him, he knows why Kojiro has been chosen: while his physical form was losing all these fights, his mind was actually storing up data on how to win. It turns out Kojiro, after being a coward as a youth, became infatuated with nature, animals and energy and began training on his own.
He perfected the art of the sword but still needed to fight the best to absorb what they knew. So he'd go on to lose many duels, whether it be to his mentor, other dojo mates or fighters across the continent, all so he could learn from them. The twist is, Kojiro actually devised methods to beat them in his mind -- he just never implemented them.
When his former master visits his hut, he finds the marks on the boards and can sense the fighters who Kojiro 'beat' in simulations. If he ever did fight them, he'd know all their moves, using his expertise to create a matrix in his mind. He could also add in further techniques and knowledge from other fighters to take on new ones.
This is why Kojiro can suss out anything a fighter can do just by seeing them step into the arena. It occurs when Poseidon enters and Kojiro reads his muscles, breathing and such. In fact, before Poseidon even throws the first blow, Kojiro has already predicted all his attacks. Every movement's been calculated, allowing him to duck, dodge and counter.
Admittedly, when Poseidon unleashes his full rage, the barrage seems like hundreds of copies of the sea god raining down blows. But it ultimately concludes with Kojiro using his double swords -- the souls of Hrist, the Valkyrie -- and thanks to one being her rage, and the other being her calm, Kojiro ends up slicing Poseidon to pieces. This shocks the gods and sends the Valhalla arena into raptures as they can tell mankind is no longer an underdog. It's all thanks to Record of Ragnarok's greatest loser.