When an opponent's monster declares an attack: Target the attacking monster; negate the attack, and if you do, inflict damage to your opponent equal to its ATK.
Rulings
Individual Card FAQs
The effect of "Magic Cylinder" negates the attack of one attacking monster, but that monster is still considered to have attacked and may not attack again or have its Battle Position manually changed.
Monsters that may attack twice during the Battle Phase, such as "Hayabusa Knight", that have their first attack negated by the effect of "Magic Cylinder" may still make a second attack.
If the attack of "Goblin Attack Force" or "Spear Dragon" is negated by the effect of "Magic Cylinder", they will not be changed to Defense Position, since Damage Calculation never occurred.
The Damage caused by this card's effect is equal to the current ATK of the attacking monster.
If an effect is chained to "Magic Cylinder" that destroys the attacking monster, or removes the attacking monster from the turn player's side of the field (including switching it to your control, or sending it to the owner's hand), no damage is dealt by the effect of "Magic Cylinder".
Netrep Rulings
"Magic Cylinder" negates the attack before it reaches the Damage Step.
"Magic Cylinder" is considered a targeting effect.
Trap|
Normal
Limited