A Flip Summon (Japanese: 反転召喚 Hanten Shōkan "Reverse Summon") is a way to Summon monsters. It can only be performed when the game state is open during your Main Phase. You simply flip a face-down monster manually into face-up Attack Position. A monster is considered to be not on the Field during the Flip Summon's Summon negation window.
A face-down monster flipped into face-up Attack Position by a card effect such as "Book of Taiyou" or "Ancient Forest" is not considered a Flip Summon.
Flip Summon does not take up your Normal Summon/Set for the turn.
You cannot Flip Summon a monster at the same turn it was Summoned, Normal Set, declared an attack (even if the attack was negated or canceled), or its battle position was changed manually that turn.