[Ka-el]

Spoiler:

So Ui Ui was the reason why Yuji swapped his soul with Kusakabe while he was training, he can teleport even just the souls of the target. And Yuji used this method twice, one with Kusakabe and the other it seems it's not revealed yet. If it's truly still untold, then it means that it will be very important for the next part of the fight.
Now Sukuna had to use binding vows to perform his domain even if not exactly as his usual ones due to his conditions (he only has two hands from the same side and very low output), but he managed to keep the range, all because Yuji's black flashes made his RCT output collapse each time he got hit. Since SUkuna is finally using Fuga, and maybe it's actually part of the Shrine but he separated it from the cutting technique to use it as an ace in the hole, then he finally has nothing more to show, especially since using binding vows for his domain will have huge drawback later, he already had to complicate a lot the use of the world cutting slash for the rest of his life to kill Gojo, who knows what complications he used to perform his DE in his conditions.
So right now the second person Yuji performed soul swapping in his training is the new "when the heck he is gonna use Fuga", now Yuji is the one who is still hiding something, Sukuna not anymore.