“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).
You can activate “Barrel Behind the Door” against Normal and Quick-Play Spell and Trap Cards, such as “Tremendous Fire” or “Attack and Receive”.
You can activate “Barrel Behind the Door” against Trigger Effects like “Lava Golem”, “Giant Germ”, “Relinquished” (when it inflicts effect damage, since “Barrel Behind the Door” can be activated during the Damage Step because it is a Counter-Trap Card), “Minar” (when discarded), “Parasite Paracide” (when it inflicts effect damage), “Bowganian”, “Mushroom Man #2”, “Reflect Bounder” (when it is attacked), “Shinato, King of a Higher Plane” (when it attacks and destroys a Defense Position monster and sends it to the Graveyard), “Granadora” (when destroyed and sent to the Graveyard), “Balloon Lizard” (when destroyed), “KA-2 Des Scissors”, “Needle Burrower”, “Atomic Firefly”, “Elephant Statue of Disaster”, and “Ameba”.
You can activate “Barrel Behind the Door” against Ignition Effects like “Cannon Soldier”, “Catapult Turtle”, “The Agent of Judgment - Saturn”, and “Chaos Emperor Dragon - Envoy of the End”.
You can activate “Barrel Behind the Door” against Flip Effects like “Princess of Tsurugi”, “Fire Sorcerer”, “Needle Ball”, “Des Koala”, and “Lady Assailant of Flames”.
You can activate “Barrel Behind the Door” against “Jigen Bakudan”. You don’t activate “Barrel Behind the Door” when the Flip Effect activates, you activate “Barrel Behind the Door” when “Jigen Bakudan” activates its effect during the next Standby Phase to inflict damage.
“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 can activate “Barrel Behind the Door” against “Ring of Destruction”, and then your opponent will take all of the damage (the damage inflicted to him and the damage that would have been inflicted to you). The same is true for “Tremendous Fire”.
You can activate “Barrel Behind the Door” against “Magic Cylinder”. The attack will be negated and the damage transferred to the opponent.
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.
You can activate “Barrel Behind the Door” during the Damage Step against appropriate effects, such as “Princess of Tsurugi”, because it is a Counter-Trap.
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.
If one player’s “Giant Germ” attacks another “Giant Germ” so that both effects activating during the Damage Step, they form a chain. If both players control “Barrel Behind the Door, only 1 player can chain theirs. The turn player will be able to chain “Barrel Behind the Door” to the opponent’s “Giant Germ” because it is Step 2 of the chain.
If “Bad Reaction to Simochi” is active, you can activate “Barrel Behind the Door” against effects that would normally cause a Life Point increase, because they inflict damage instead.
If you re-direct damage to your opponent using “Barrel Behind the Door”, and you have “Dark Room of Nightmare” on the field, then “Dark Room of Nightmare” inflicts 300 more damage.
For effects like “Princess of Tsurugi” “Thunder Crash”, and “Restructer Revolution”, where damage is added up, “Barrel Behind the Door” re-directs the entire sum of damage.
Netrep Rulings:
“Barrel Behind the Door” can only be used against an effect that will definitely inflict Damage to you.
“Barrel Behind the Door” may be chained to your own effect that inflicts Damage to you.
“Barrel Behind the Door” may be used in the Damage Step.
“Barrel Behind the Door” may not be used against “Time Wizard” or “Dice Jar” because there is no guarantee that you will be receiving Damage.
“Barrel Behind the Door” may not be used against Continuous effects, including “Gravekeeper’s Vassal” or “Wave-Motion Cannon.”
“Barrel Behind the Door” may not be chained to an effect that causes you to lose life such as that of “The Immortal of Thunder.”
“Barrel Behind the Door” may not be used against effects that redirect Damage, such as that of “Barrel Behind the Door.”
“Barrel Behind the Door” may only be used against one instance of Damage being inflicted to the player controlling “Barrel Behind the Door.”
“Barrel Behind the Door” may be chained to the Damage inflicted as a result of “Granadora’s” effect.