This card can be activated during the Damage Step, before Damage Calculation.
If a monster affected by "Shrink" is turned face-down and face-up again that turn, the effect of "Shrink" is no longer applied.
Even if a monster is targeted by multiple copies of "Shrink", its ATK will still be half of its original ATK. For example, if a monster with 1400 original ATK is affected by 2 copies of "Shrink", its ATK will be 700 NOT 350.
The effect of "Shrink" works like "Megamorph", setting a new current ATK based on original ATK. If the same monster is targeted by both "Shrink" and "Megamorph", only the effect of the card that resolved last will be applied.
If a monster's ATK is modified by a Field or Equip Spell Card, or a Continous Effect of a monster or Trap Card, etc., and it is targeted by "Shrink", first calculate half of its original ATK, then add the appropriate modifiers from the Equip Spell, etc.
"Shrink" has no effect on monsters whose original ATK is indefinite and whose effects don't set an original ATK, like "Maju Garzett," "Option" and "Gren Maju Da Eiza".
For monsters like "Chimeratech Overdragon", "Megarock Dragon" and "UFOroid Fighter" whose original ATK is set when they are Summoned, "Shrink" will halve that value.
If a monster like "King of the Skull Servants", whose original ATK varies, is affected by "Shrink", its ATK will become half of its original ATK at the time "Shrink" resolved. If its ATK changes by its own effect in the same turn, recalculate the effect of "Shrink" based on the new original ATK.
If "Shrink" affects a monster whose ATK is doubled by "Limiter Removal", that monster's ATK will become half of its original (printed) ATK.
If "Bottomless Trap Hole" is activated in response to a Summon and "Shrink" is chained so that the monster's ATK is less than 1500 when "Bottomless Trap Hole" resolves, that monster will not be destroyed and removed from play by "Bottomless Trap Hole".
If a monster affect by "Shrink" attacks while "Mirror Wall" is in play, both effects are applied and the monster's ATK becomes 1/4 its original ATK. Because "Mirror Wall" continually recalculates, the result is the same no matter which card is activated first.
If a monster whose ATK is reduced by "Dark Jeroid" is affected by "Shrink", first calculate half that monster's original ATK, then subtract 800 for "Dark Jeroid's" effect; this becomes the monster's current ATK.
If "Exarion Universe" is targeted by "Shrink", its ATK will become half of its original ATK, even if its effect to reduce its own ATK by 400 has been used.