It generates based off of orange attacks and damage taken iirc. You can also charge it manually by sacrificing half your HP then triggering it on 2 moves that are orange. Each of those cost 3 bars, one is a cinder/ash move and the other is an air grab where you slam the opponent on the ground 3 times before they explode. And that's if they're at minimum 6 stacks. Burst can also trigger the nuke IF they're at 8 stacks