From the Super Mario Wiki
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Mario initiates the move by lighting the ball on fire and dribbling it in a vertical M shape. Once initiated, Mario will throw the ball with one hand towards the hoop with red flames trailing behind it. If shot from inside the 3-point line, the ball will land in the hoop with a fiery explosion.
If shot from outside the 3-point line, the visuals don't change, but a teammate needs to grab the ball and dunk it in.