What it does
I'm not aware of "Dragonfire Dive" as an official Magic: The Gathering keyword or ability. It doesn't appear in any rules set or card I can verify, so I don't want to invent mechanics that might mislead you at the table.
It's possible you're thinking of a similarly-named ability, a card with "Dragonfire" in its title (like Dragonfire Burst or various dragon-themed cards), or a mechanic from another game or a custom/unofficial set. Keyword abilities like Dethrone, Devour, or the dragon-flavored Battalion and Exert get mixed up easily.
If you can tell me the exact card name or where you saw "Dragonfire Dive," I can pull up the real rules text and explain how it works and where it shines in Commander. Accurate info beats a confident guess here.
Iconic cards with Dragonfire Dive
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