Jujutsu Kaisen is one of shonen's biggest titles right now, alongside its competitors Demon Slayer and My Hero Academia, and it also features one of shonen's best combat systems. In the Jujutsu Kaisen universe, negative human emotions can coalesce into powerful supernatural monsters known as curses, and only jujutsu sorcerers who use their own cursed energy can fight back.
Early in the series, the kind protagonist Itadori Yuji learned to master his emotions so he can better control his cursed energy, and many of his allies are doing the same, channeling their negative emotions into combat energy. Interestingly, this negative combat system could lead to a self-sabotaging paradox, and jujutsu sorcerers have found a way around it. Emotions are a tricky thing.
When Negative Emotions Lead To Victory, Then Defeat
Jujutsu Kaisen's combat system is a fickle one, being based on human emotions. Early on, Yuji had to watch The Lord of the Rings and restrain his emotions just so he can use cursed techniques at all, let alone master them, and it took him many hours to reach that point. What's more, many of Jujutsu Kaisen's characters are adolescents, and everyone knows that adolescents are emotionally volatile people who are still figuring themselves out and finding their place in the world. Their strong emotions can lead to powerful jujutsu techniques, such as Megumi's summoned animals or Nobara's hammer and nails, but this can also backfire if the sorcerer bases their emotional power on the wrong topic. For example, sorcerers must not derive their power from the pain of defeat.
At first, a humiliating defeat seems like the perfect fuel for a sorcerer. That sorcerer can channel the rage and humiliation of defeat and become stronger in the next fight to even the odds, but then a paradox arises. When the sorcerer claims victory, they are filled with positive emotions of elation, satisfaction and pride, and the negative emotions that got them to that point are lost. This may weaken the sorcerer and lead to defeat in the next battle if they keep this up. Then they began again, going back and forth with no way to stabilize or get ahead in their training -- or, they and a bitter rival may go back and forth, trading defeats and the negative power that arises from those losses. Competitive and insecure young sorcerers could potentially fall victim to this trap and struggle to break free.
Another shonen series demonstrated this: the classic series Ranma 1/2. At one point in the story, a down-in-the-dumps character named Ryoga learned the Shishi Hokodan technique, which uses negative emotions as power, just like Jujutsu Kaisen sorcery, and used it to defeat his rival, the protagonist Saotome Ranma. The elation of victory reduced Ryoga's power, and then Ranma used the same technique to get even, fueled by his humiliating defeat. For comic effect, the two characters went back and forth in this endless paradox until Ranma eventually used a different move to defeat Ryoga for good. Theoretically, something like this could happen in Jujutsu Kaisen, but as the main heroes have already shown, there are more viable alternatives that don't lead to frustrating paradoxes.
How Yuji & His Friends Channel Their Negative Emotions
Yuji and his classmates channel negative emotions all the time to fuel their techniques, and so far, they have avoided the victory-defeat cycle that could arise from fueling themselves from previous losses. Instead, these trainee sorcerers can base their negative emotions on something more fixed and permanent, such as grief over someone passing away, frustration at the unfairness of the world and more. These aren't pleasant topics, but they are ideal for sorcery and don't lead to paradoxes. A sorcerer like Yuji, Nobara or Megumi can base their powers on such topics to keep their power levels consistent and stable, since the sources of their power are similarly stable. A character like Zenin Maki won't let the elation of victory wipe away her frustration at her family, and the elation of victory won't easily negate Yuji's pain after seeing Junpei perish at Mahito's hands.
These young sorcerers understand that selfish negative emotions will only sabotage them in their battles against curses, and they are choosy about the basis of their negative feelings and sorcery. They need something sustainable and long-term to keep them fueled, which helps compensate for the volatile and unpredictable nature of their own emotions. Adolescent sorcerers like Maki, Nobara and Megumi are clearly self-aware of their teenage angst, and they have no choice but to weaponize it properly or get trapped in an endless loop that makes them vulnerable to curses. Figuring this out can't be easy for these sorcerers, but it's vital if they intend to save the world from the likes of Mahito, Hanami and Jogo.
