Just some info if you weren’t already aware:
Sukuna’s flame ability is called furnace, in the manga the name is revealed. Fuga means open, so what he said was “furnace open”. Now I don’t know if you are following the manga step for step or doing everything the way it is done in canon JJK, but the way the fire arrow (furnace) works is that the furnace needs to be heated up sufficiently before it can be opened. And the way that the furnace is heated up is by using cleave and dismantle to slash and cut things and people. The more you cut, the hotter the flames of the furnace get, until eventually they are hot enough to be unleashed.
Cleave requires physical contact but can slice through virtually anything no matter how tough it is.
Dismantle is launched at range and its attack power is determined by Sukuna’s cursed energy output levels.
Sorry for the essay lol, I just see a lot of people who don’t know these things. Im also just a bit of a Yapper in general, can’t help myself from yap-yappity-yapping at times.