Problem with undercombos

[alfinchkid]

I'm making a side-quest place called "Upgrade Dungeon", where you find upgrades to the rings, wand, candle, boomerang, and arrow. In the first room is the Red Candle. I have it behind trees that you must burn down via blue candle. Undre these trees I have the undercombo of a regular wal tile (tile 1). I test it, and the trees burn, but then I have a gray strip. Thinking I'll just go back and fix it, I try to walk over it. I can't. I go back to editor, and it shows the undercombo as the tile I want it to be. What's going on?

[Nimono]

I'm making a side-quest place called "Upgrade Dungeon", where you find upgrades to the rings, wand, candle, boomerang, and arrow. In the first room is the Red Candle. I have it behind trees that you must burn down via blue candle. Undre these trees I have the undercombo of a regular wal tile (tile 1). I test it, and the trees burn, but then I have a gray strip. Thinking I'll just go back and fix it, I try to walk over it. I can't. I go back to editor, and it shows the undercombo as the tile I want it to be. What's going on?
You're trying to use an undercombo as a SECRET combo. In short, if you're triggering FLAGS, place the SECRET COMBO under the item you're using. For the bushes, place the tile under either Blue or Red Candle.

[firefly]

In other words, if you want to burn a tree via flags, to make the undercombo for the bushes enter the secret combos menu, select burn and you'll see some squares, those are the "under combos" for these flags.