Exert

Exert is a keyword action introduced in Amonkhet. Exert is focused in white, red and green.

Description
When a creature with the exert ability is exerted, then an ability is triggered and the creature doesn't untap at the beginning of its controller's next untap step.

For easy play, extra "exerted" markers were provided on punch cards in the Amonkhet set.

The flavor of this mechanic is that you have the ability to exert your power over the creatures, making them push themselves a little harder than they should. If the creature exerts, it means the creature is expending a large amount of effort, resulting in a bonus. As a result, the creature is then exhausted by the effort, and thus it doesn't untap in the next untap step.

In Hour of Devastation the mechanic was tweaked. Instead of being an attack trigger, these creatures exert themselves (aka choose to not untap on the next turn) when they use a specific activated ability.

History
It was created when Jackie Lee suggested to change the sacrifice drawback of reckless to a less egregious one like not untapping next turn. Exert counters were considered by R&D but eventually rejected.

Rulings

 * All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.
 * Some cards in the Hour of Devastation set that let you exert a creature let you do so as a cost of an activated ability.
 * If a creature has a targeted triggered ability that triggers when you exert it, you can exert it even if there isn't a legal target for that triggered ability.
 * Some cards have abilities that trigger whenever you exert any creature. These abilities trigger when you exert that creature or any other creature you control.
 * You can't exert a creature unless an effect allows you to do so. Similar effects that "tap and freeze" a creature (such as that of Decision Paralysis) don't exert that creature.
 * If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert's effect preventing it from untapping expires without having done anything.
 * If you gain control of another player's non-exerted creature until end of turn and exert it, it will untap during that player's untap step.