So, idk if you understood, but Yuuji discovered that negative emotions are what's needed to actually use curse power, since that's where curses in general come from and what they essentially feed off of. Yuuji channeled all the negative emotions he was feeling and put that into his "fist." It probably would've been more powerful if he had his fingers.
Also, my guess is that because of all the stress he just went through, and having had a hand and fingers cut off, he probably completely passed out/clocked out after switching. And when he wakes up, it might be harder tryna get his body back now that Sukuna got another one of his fingers back, making him more powerful.