Barrel Behind the Door
Counter Trap
When an effect that inflicts damage to Life Points is activated (except Battle Damage), switch the damage you receive to your opponent's Life Points.
Seems simple, but I have seen the strangest stuff in regards to this card. I have seen people try and use cost effects and reflect them back at people (That is a No No) I have seen people use them against Continuous effect cards Like Wave Motion Cannon (Another No NO) and I have seen people chain 2 barrels to each other (No...bad...), and against Dice Jar.(-_-). This card has been a serious pain not just for the ruling world but for anyone trying to use this card. Recently Kevin got around to posting the rulings for PGD and joy swept the land for now we could point to the UDE FAQ and say this is where we got the answer from.
Is it still a confusing card? At times yes, I still run into people who as about Wall of Revealing Light and Barrel behind the door. But Thankfully more people are using Hallowed Life Barrier and Pikeru's Circle of Enchantment over this. Its still a nice card, especially since Burn Decks are popular now. As long as you understand the basic rulings on this card.
"Barrel Behind the Door" can only be activated against an effect that inflicts damage to YOUR Life Points.
"Barrel Behind the Door" can only be activated against DAMAGE. It cannot be activated against costs like "Imperial Order" or "Wall of Revealing Light". It cannot be activated against effects that cause you to lose Life Points like "The Immortal of Thunder". It cannot be activated against effects that switch damage, like another "Barrel Behind the Door", because they do not inflict damage, they simply transfer it.
You cannot activate "Barrel Behind the Door" against a Continuous Spell or Trap Card, an Equip Spell Card, or a Field Spell Card. So you cannot activate it against "Magical Thorn", "Dark Snake Syndrome", "Wave-Motion Cannon", or "Black Pendant".
You cannot activate "Barrel Behind the Door" against a Continuous Effect Monster like "Gravekeeper's Vassal" or "Fire Princess" (even if an effect like "Gift of the Mystical Elf" is activated while "Fire Princess" is on the field).
"Barrel Behind the Door" cannot be activated against cards that do not inflict damage when the card resolves, such as "Graverobber", which inflicts damage later.
You cannot activate "Barrel Behind the Door" against effects if you don't know for certain that they will inflict damage when the effect is activated. So you cannot activate it against "Dice Jar", "Judgment of Anubis", "Time Wizard" or "Ante". Your cannot activate "Barrel Behind the Door" against "Ceasefire" if all monsters on the field are face-down, or if all monsters on the field are face-up Normal Monsters, but you can activate it against "Ceasefire" if there is at least 1 face-up Effect Monster on the field.
Remember that the effects that Counter-Traps are chained to must immediately precede them in the chain. So if you activate "Ring of Destruction", and intend to chain "Barrel Behind the Door" to it, if your opponent chains something in-between (even an unrelated effect like "Jar of Greed"), then you cannot chain "Barrel Behind the Door" because "Ring of Destruction" is no longer the preceding step in the chain.
Simple Right?