Warning: the following article contains spoilers for Jujutsu Kaisen Chapter 172 "Tokyo No. 1 Colony, Part 12," by Gege Akutami, John Werry, and Snir Aharon, available in English from Viz Media.
This fight between Fushiguro and Reggie has been a battle of wits for the most past rather than a contest of strength. There's no doubt that they both have strong Cursed Techniques, but strength isn't what this battle comes down to. When it's all said and done, the victor is the one who better understands how to use their own Cursed Technique in unconventional ways while deciphering how their opponent's technique works. Moreover, deception and manipulation play important roles in deciding who survives and who dies.
It's an interesting contrast to Yuji's fights because while he obviously has to figure out how his opponent's Cursed Technique works, he doesn't have one of his own. This means that there are fewer mind games involved. Fushiguro and Reggie have been constantly demonstrating how tricky they can be throughout their fight. Fittingly, what determined the winner of this struggle was literally one's ability to mislead the other.
Fushiguro and Reggie both placed unbearably heavy weight on each other at the end of the previous chapter as the fight became a competition to see who'll get crushed first. They're both able to withstand the weight due to them reinforcing their bodies with Cursed Energy. Reggie considered going inside Fushiguro's shadow in order to be able to use his trump card but thinks better of it as he didn't know what was actually in there. He had no idea if he'd even be able to breathe within the shadow or get out if Fushiguro ended up dying while he's in there.
Reggie attempted to use one of his receipts but failed as he immediately succumbs to the weight of Max Elephant. Fortunately for him, objects he conjures with his technique disappear after he gives them a command. The cars he made sink into the shadow in Chapter 171 are still there since the entire point was to force the weight on the Fushiguro. Thus, he commanded one to bring him up to the surface.
Reggie then used his Cursed Technique to conjure an entire house to drop on Fushiguro. Fushiguro quickly went into his shadow before it could land on him and dismissed his Domain which causes the house to destroy the floor of the gymnasium they were on. The floor had a pool below it which Fushiguro pulls Reggie down in while still bearing the weights that were placed on him. This forced Reggie to release his technique. It's also to Fushiguro's advantage that the receipts got wet since he realized in Chapter 170 that water's their weakness.
Fushiguro believed that Reggie couldn't use his receipts if they got wet but that's not quite right. What mattered was whether or not the print on the receipts disappear. If the print hasn't disappeared then the receipts can still be used. However, if the receipt was wet, the condition to burn up the receipts with Cursed Energy can't be met. Thus, Reggie was unable to use his Cursed Technique.
Fushiguro proved that he'd been thinking two steps ahead of Reggie the entire time as he tried to deceive him into believing that he can no longer use his Shikigami, so they'd be engaging in a fistfight from this point on. Reggie didn't fall for it but wanted to deceive Fushiguro into thinking he did and deduced that he was waiting to use any Shikigami besides his Divine Dog. However, the assumption that he wouldn't use Divine Dog was a deadly mistake and exactly what Fushiguro wanted. He had been pretending that he could no longer use Divine Dog since it was stabbed in Chapter 167.
Divine Dog has been fine this entire time. He simply hasn't been using it so that his deception would be believable. As the two began their hand-to-hand combat, Fushiguro wasted no time in having Divine Dog bite off a huge chunk of Reggie's shoulder and the side of his head. The fight appears to be over and it seems safe to assume that Fushiguro will gain more Culling Game points. The main reason he won is because of his cleverness, not his powerful Cursed Technique and Domain Expansion
