Trueheart Twins

If you gain control of another player's creature until end of turn and exert it, it will untap during that player's untap step.

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.

All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. 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.