Transform

Transform is a triggered/activated* ability that allows you to turn double-faced cards upside down.

* Note: when Transform was introduced there were only cards with the triggered version available, although other upcoming sets might include activated versions as well.

Cards with transform can always be recognized by an icon:
 * Ixalan: Front: Compass rose, Back: Land symbol

Expansion Info
Enrage was introduced into Magic Arena with Ixalan (closed beta).

Video
Watch this explanatory video to see how this game concept works.

Comprehensive Rules
701.26. Transform

701.26a To transform a permanent, turn it over so that its other face is up. Only permanents represented by double-faced cards can transform. (See rule 711, “Double-Faced Cards.”)

701.26b Although transforming a permanent uses the same physical action as turning a permanent face up or face down, they are different game actions. Abilities that trigger when a permanent is turned face down won’t trigger when that permanent transforms, and so on.

701.26c If a spell or ability instructs a player to transform a permanent that isn’t represented by a double-faced card, nothing happens.

701.26d If a spell or ability instructs a player to transform a permanent, and the face that permanent would transform into is represented by an instant or sorcery card face, nothing happens.

701.26e Some triggered abilities trigger when an object “transforms into” an object with a specified characteristic. Such an ability triggers if the object transforms and has the specified characteristic immediately after it transforms.

701.26f If an activated or triggered ability of a permanent that isn’t a delayed triggered ability of that permanent tries to transform it, the permanent transforms only if it hasn’t transformed since the ability was put onto the stack. If a delayed triggered ability of a permanent tries to transform that permanent, the permanent transforms only if it hasn’t transformed since that delayed triggered ability was created. In either case, if the permanent has already transformed, the instruction to transform is ignored.

711. Double-Faced Cards

711.1. A double-faced card has a Magic card face on each side rather than a Magic card face on one side and a Magic card back on the other. Each face may have abilities that allow the card to “transform,” or turn over to its other face. Tokens and cards with a Magic card back can’t transform. (See rule 701.26, “Transform.”)

711.1a A double-faced card’s front face is marked by a front-face symbol in its upper left corner. On Magic Origins double-faced cards, the front-face symbol is a modified Planeswalker icon. On cards in the Innistrad block and Shadows over Innistrad set, as well as on Ulrich of the Krallenhorde in the Eldritch Moon set, the front-face symbol is a sun. On other Eldritch Moon double-faced cards, the front-face symbol is a full moon. On Ixalan cards, the front-face symbol is a compass rose.

711.1b A double-faced card’s back face is marked by a back-face symbol in its upper left corner. On Magic Origins double-faced cards, the back-face symbol is a full Planeswalker icon. On cards in the Innistrad block and Shadows over Innistrad set, as well as on Ulrich, Uncontested Alpha in the Eldritch Moon set, the back-face symbol is a crescent moon. On other Eldritch Moon double-faced cards, the back-face symbol is a stylized image of Emrakul. On Ixalan cards, the back-face symbol is a land icon.

711.1c If the back face of a double-faced card is a creature, the front face of that card will have the back face’s power and toughness printed in gray above the power and toughness box. This is reminder text and has no effect on game play.

711.1d Meld cards have a Magic card face on one side and half of an oversized Magic card face on the other. These aren’t double-faced cards and are subject to their own set of rules. See rule 712, “Meld Cards.”

711.2. Players who are allowed to look at a double-faced card may look at both faces.

711.3. Players must ensure that double-faced cards in hidden zones are indistinguishable from other cards in the same zone. To do this, the owner of a double-faced card may use completely opaque card sleeves or substitute a checklist card (see rule 713). Sanctioned tournaments have additional rules for playing with double-faced cards. See rule 100.6.

711.4. Each face of a double-faced card has its own set of characteristics.

711.4a While a double-faced card is outside the game, in a zone other than the battlefield, or on the battlefield with its front face up, it has only the characteristics of its front face.

711.4b While a double-faced permanent’s back face is up, it has only the characteristics of its back face. However, its converted mana cost is calculated using the mana cost of its front face. If a permanent is copying the back face of a double-faced card (even if the card representing that copy is itself a double-faced card), the converted mana cost of that permanent is 0.

711.5. Only permanents represented by double-faced cards can transform. (See rule 701.26, “Transform.”) If a spell or ability instructs a player to transform any permanent that isn’t represented by a double-faced card, nothing happens.
 * Example: A Clone enters the battlefield as a copy of Wildblood Pack (the back face of a double-faced card). The Clone will be a copy of the Wildblood Pack. Because the Clone is itself not a double-faced card, it can’t transform.
 * Example: A player casts Cytoshape, causing a Kruin Outlaw (the front face of a double-faced card) to become a copy of Elite Vanguard (a 2/1 Human Soldier creature) until end of turn. The player then casts Moonmist, which reads, in part, “Transform all Humans.” Because the copy of Elite Vanguard is a double-faced card, it will transform. The resulting permanent will have its back face up, but it will still be a copy of Elite Vanguard that turn.

711.6. If a spell or ability instructs a player to transform a permanent, and the face that permanent would transform into is represented by an instant or sorcery card face, nothing happens.

711.7. If a double-faced card is cast as a spell, it’s put on the stack with its front face up. See rule 601, “Casting Spells.”

711.8. A double-faced card enters the battlefield with its front face up by default. If a spell or ability puts it onto the battlefield “transformed,” it enters the battlefield with its back face up.

711.8a If a player is instructed to put a card that isn’t a double-faced card onto the battlefield transformed, that card stays in its current zone.

711.9. If an effect allows a player to cast a double-faced card as a face-down creature spell, or if a double-faced card enters the battlefield face down, it will have the characteristics given to it by the rule or effect that caused it to be face down. That card remains hidden, using either a face-down checklist card or opaque sleeves. See rule 707, “Face-Down Spells and Permanents.”

711.9a While face down, a double-faced permanent can’t transform. If it is turned face up, it will have its front face up.

711.10. Double-faced permanents can’t be turned face down. If a spell or ability tries to turn a double-faced permanent face down, nothing happens.

711.11. A double-faced card that is exiled face down remains hidden, using either a face-down checklist card or opaque sleeves. See rule 713, “Checklist Cards.”

711.12. When a double-faced permanent transforms, it doesn’t become a new object. Any effects that applied to that permanent will continue to apply to it after it transforms.
 * Example: An effect gives Village Ironsmith (the front face of a double-faced card) +2/+2 until end of turn and then Village Ironsmith transforms into Ironfang. Ironfang will continue to get +2/+2 until end of turn.

711.13. If an effect instructs a player to choose a card name, the player may name either face of a double-faced card but not both.