Game mechanics
Game mechanics refer to any action that automatically happens due to the interior workings of the Yu-Gi-Oh! game.
Using Cards and the Graveyard
Whenever a card is Destroyed or Discarded, when a Normal or Quick-Play Spell is used, or when a Normal or Counter Trap is used, it is always sent to the Graveyard unless a card effect states otherwise, such as "Banisher of the Radiance" or "Macro Cosmos."
Field Spell Cards
Only one Field Spell Card can be on the field at a time. If a player has one on the field and the other player Activates a second one, when the second one's Activation resolves, the first one is automatically destroyed. Because this is a Game Mechanic and not a card effect, cards like "Magical Citadel of Endymion" are not protected from this destruction. However, if the second Field Spell Card is destroyed before its Activation resolves, such as with "Mystical Space Typhoon", the original one is not destroyed.
The player who controls the Field Spell Card is also allowed to play a different one. In this situation, however, the first one is destroyed and sent to the Graveyard before the second one is activated. This creates a minor gap of time in which there is no Field Spell Card on the field, a large enough gap to destroy cards such as the Earthbound Immortals who cannot survive without the presence of one.
Equip Cards
Equip cards continuously target the equipped monster to make sure it is still there. If that monster is no longer face-up on the field, the Equip card is automatically destroyed. This can be due to either being destroyed or removed from the field by battle of card effect, or simply being flipped face-down with "Book of Moon".
Additionally, Equip cards also continuously target to make sure the equipped monster fits its requirements, and if it suddenly no longer meets its requirements, the Equip card is destroyed. If "Machina Peacekeeper" is equipped to a "Machina Fortress" and "Zombie World" is Activated, "Machina Peacekeeper" is automatically destroyed because "Machina Fortress" has been changed into a Zombie-type and no longer meets the Machine-type requirement.
