The following article contains spoilers for Jujutsu Kaisen Chapter 187 "Tokyo No. 2 Colony, Part 6," by Gege Akutami, John Werry and Snir Aharon, available in English from Viz Media.
Kinji Hakari is a character who has been spoken of throughout the series by several key characters long before his first appearance. Gojo made it clear that he believes Hakari is one of his most powerful students with the potential to rival his own strength someday. Even Yuta claimed that Hakari is stronger than him. This is noteworthy, since Yuta is a special grade sorcerer second only to Gojo, confirmed as recently as Chapter 173.
At the time, Maki denied what Yuta said about Hakari's strength. Based on how Hakari is faring in his fight against Hajime Kashimo, it's now safe to say Yuta wasn't being modest. Hakari is proving himself worthy of Gojo and Yuta's praise due to his incredibly powerful and tricky Cursed Technique, Private Pure Love Train. Hakari is having no problems at the moment and definitely has the upper hand, which is surprising, considering how Kashimo destroyed Panda. Interestingly enough, Kashimo doesn't seem particularly worried.
Hakari's Cursed Technique, and by extension, his Domain Expansion, are based on gambling. Private Pure Love Train, his Cursed Technique, allows him to manipulate probability and use attacks based on the manga it's named after.
So far, Hakari has primarily used the technique through his Domain Expansion, Idle Death Gamble. Idle Death Gamble works similarly to pachinko, allowing Hakari to gamble for the chance to win a jackpot, which would temporarily grant him a bonus of unlimited Cursed Energy.
When Hakari hits the jackpot and gains unlimited Cursed Energy, the theme song for Private Pure Love immediately starts playing and lasts four minutes and eleven seconds. During that time, the unlimited Cursed Energy allows his body to reflexively perform Reverse Cursed Technique to prevent his body from breaking. The most fascinating thing about this is that Hakari never learned Reverse Cursed Technique. This means that for the song's duration after hitting a jackpot, Hakari is essentially immortal.
Hakari can also open his Domain multiple times as long as he keeps hitting the jackpot. This is because during a round of Idle Death Gamble in which he hits the jackpot, his Cursed Energy and expended Cursed Technique replenish. At one point during the fight, Kashimo completely decapitates one of Hakari's arms, but it doesn't affect him at all and his arm quickly regenerates. It's a testament to how powerful Hakari's technique is.
Kashimo, to their credit, fully understands Hakari's Cursed Technique, but manages to stay calm. Even though Hakari definitely has the advantage, it doesn't feel like Kashimo is losing the fight. The ancient sorcerer just keeps getting more excited and insists that he'll kill Hakari, despite his immortality. It could be that Kashimo hasn't shown his true power yet. It's very likely they'll cast their own Domain Expansion, assuming they have one. In other words, they might still be able to turn the tables on Hakari.