Thank you sensei for explaining it so thoroughly.A simple but useful tech. I think this is one of those techs where it takes time to figure out that its an illusion. The user creates an illusion where the opponent sees a fireball descending from the sky upon them. Normally in an illusion when you see something approaching you, like the clone in Servant tech, you can quickly spot that its a Gen by deducting that the clone has no shadow cast on the ground. But that reasoning won't work in the case of the fireball. The fireball is nowhere near the ground but its descending from the sky. In case of shadows, higher an object is from the ground, further away is its shadow from the point where it will land. So until the fireball comes near the ground(and the opponent), the shadow reasoning cannot be used. This technique fails badly when the battle is going on water or ice surface cause the moment the opponent sees the fire ball has no reflection in the water or on the ice surface, he will know its not real. The opponent can figure out its a Gen when the fireball comes near him and he can't feel the heat of the fireball. But that's actually time consuming. The opponent can figure out its a Gen if he doesn't hear the sound of flames. If there is wind blowing in the terrain and when the fireball descends but you can't see the flames blowing a little in the direction of the wind, then its not real but an illusion. A cool way to use this is just after using a multiple projectile fire tech. I'd first use a projectile where after firing multiple shots at my opponent, I'd shot one in the sky and then would bluff "How do you plan on taking care of the huge fireball that will hit you from above and burn you to ashes?" Then I'd use this tech to fool my opponent. One can come out of this tech by causing themselves a minor injury like a small slash in the skin.
Regarding the jutsu itself, there's not much I can really add. Maybe a cool use would be a combination with the ( Katon: Gōryūka no Jutsu ) - Fire Release: Great Dragon Fire Technique that is used to prepare Kirin. You shoot the fireballs into the sky and then you follow up with this technique, making it seem as they descend. While in the air you sort of make them "merge" together so that the shooting into the sky actually makes some sense and this way your opponent see's the fireball while in reality you're setting up Kirin.