What it does
Burn Up is a Fire-type special move that deals Fire-type damage. After it hits, the user permanently loses its Fire typing for that battle. A pure Fire-type becomes typeless ('???'), a dual-type loses the Fire half and keeps the other. Switching out does not restore the typing.
If the user has no Fire typing, the move fails entirely.
Losing Fire typing removes the Water, Rock, and Ground weaknesses — but also removes the Fire resistances and any type-based interactions tied to being Fire. Burn status damage is not affected (burns don't care about your type). The main competitive value is stripping the vulnerability to common coverage types.
Who runs it
Typhlosion runs it on about 16% of its Champions sets. Typhlosion is pure Fire — after Burn Up, it becomes typeless and loses its three main weaknesses. With decent Special Attack and Speed, buying extra turns by removing those weaknesses is the core reason to slot this in.
One thing to track: after Burn Up, Typhlosion no longer has any type at all. Any ability interactions or damage modifiers that depended on it being Fire — like Flash Fire's activation if targeting a Fire-type — are gone.
How to use it
The right moment depends on whether the typing change matters in the current matchup. If the opponent has strong Water or Ground coverage, using Burn Up early means those moves hit for neutral damage going forward instead of landing a 2x shot. In doubles, where spread Water moves can hit everything on your side at once, this change can be the difference between Typhlosion surviving a turn it otherwise wouldn't.
If the opponent doesn't have relevant super-effective coverage, there's no urgency — deal damage first, and consider Burn Up later if the board situation changes.
Tips & strategy
The part most players overlook: losing Fire typing also removes the Fire resistance. Grass was 0.5x against Fire; after Burn Up, Grass hits for neutral damage. Check whether opponents have Grass-type attackers like Rillaboom whose damage profile changes after the typing swap.
If the typing change meaningfully extends Typhlosion's time on the field, use it. If the matchup doesn't have relevant coverage, it's just a Fire-type damage move — the typing consequence is irrelevant. Don't burn the typing away for no reason.