Copied from Yu-Gi-Oh! Wikia
The effect of "Piercing" is characterized by the following text in the card lore:
- Example: Player A's "Dark Driceratops" (ATK 2400) attacks Player B's Defense Position "Mystical Elf" (DEF 2000). Regardless if Mystical Elf is ed or not, Player B takes the difference (400 points in this case) as .
Although "Piercing" is the official term for this scenario in the context of Yu-Gi-Oh!, in many circles of duelists, Piercing damage is often colloquially referred to instead as "Trample", borrowing from the popular and longer-established Magic: The Gathering card game.
An interesting side-effect of Equip Spell Cards that give Piercing, is that you can inflict damage to your opponent by equipping them to your opponent's monster(s). Then, when your opponent attacks one of your Defense Position monsters with an ATK greater than your monsters DEF, he/she will take the difference. After all, even though it's your opponent's monster, it's your Spell Card, and thus it's your opponent — not you — who takes the extra damage.