Young Link's Kokiri sword makes fire?

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I have witnessed a rare case of Young Link's down air attack doing fire damage, and propelling his victim downward instead of upward. The blow was quite strong, though it just "grounded" the victim.
My questions are as follows:
- How is this blazing hit triggered?
- Is it ever useful (in either recreational or competitive play)?
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Glitch Infinite - Leon (Lucina) Vs. Skittles!! (Young Link) - SSBU Ultimate Tournament
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Images: Malibi 75, Malibi 75, Tima Miroshnichenko, Tima Miroshnichenko