"'Source: Yugioh Wikia'"
Flip monsters (Japanese: リバースモンスター Ribāsumonsutā "Reverse Monster"), formerly Flip Effect Monsters (リバース
Previously, Flip Effects (as they were capitalized at the time) were a type of card effect like Trigger and Ignition Effects, with Flip Effects being able to be identified by the text "FLIP:" at the start of the effect; Flip Effect Monsters were simply monsters that had a Flip Effect. Starting in Starter Deck 2014 (in the OCG) or Super Starter: Space-Time Showdown (in the TCG), Flip is treated as an ability and the word "Flip" is written on the cards' Type/Ability line.
A Flip effect is a type of Trigger Effect that is preceded by "FLIP:". This kind of effect meets its activation timing when the Flip monster on the field is flipped from face-down to a face-up, even during the Damage Step.
Some monsters have effects with the timing "when this card is flipped face-up" (unofficially called Pseudo-Flip Effects), such as "Snowman Eater", which, while mechanically identical, are distinct from Flip effects.
When a face-down Flip monster is attacked, during damage calculation the monster is flipped face-up, then it is determined whether or not the monster is destroyed by battle. During the Damage Step, after damage calculation but before being sent to the Graveyard if it was destroyed by battle, the Flip monster's Flip effect activates and resolves on the field, if the timing is correct.