There was actually a source for it in the Original Work, if you looked hard enough.
That was Naruto's Senjutsu: Lava Release: Rasenshuriken, which he performed with the help of the Four-Tails' Lava Release Chakra.
Naruto had added Senjutsu and Lava Release—a fusion of Fire Attribute and Earth Release—to the Rasenshuriken. The technique had the power to burn, slice, and explode.
This was already beyond the realm of Forbidden Techniques. Even Senjutsu wasn't this outrageous. This was straight-up cheating.
It had no value as a reference.
In theory, the Rasengan could be developed with any chakra nature.
Lightning, Fire, Earth, and Water were all possible.
Liao Wu chose Fire.
What could Fire do?
Fire could burn, release intense heat, and explode.
Liao Wu focused mainly on its explosive properties. First, he had to infuse Fire Attribute Chakra into the Rasengan, then endure the intense heat within and control its terrifying explosive force.
Finally, he had to figure out how to throw it.
This was an explosive Rasengan, similar in nature to the Wind Release: Rasenshuriken. It could affect its user, so he had to find a way to throw it.
That required even greater chakra control.
Drawing on his experience with the Spore Clone, Liao Wu slowly experimented.
A blue-white sphere of swirling light formed in Liao Wu's hand.
This was the normal Rasengan.
Then Liao Wu infused it with his Fire Attribute Chakra.
The blue-white color inside the Rasengan turned fiery red, making it look like a rapidly spinning fireball.
It radiated terrifying heat and astonishing explosive force.
The Spore Clone had already taken shape. All that remained was for Liao Wu to gradually refine it.
The entire room grew scorching hot. Sweat streaming down the wall, Obito stared in horror at the Fire Style Rasengan in Liao Wu's