Double — Riftbound Rules

Doubling is the act of increasing a numeric attribute by an amount equal to that attribute’s current value.

Section 433 — 5 rules

433.1.
Doubling is the act of increasing a numeric attribute by an amount equal to that attribute’s current value.
433.1.a.
This creates an effect that modulates that attribute by that specific amount for the duration specified by the Game Effect that instructed the player to perform this action. Example: A unit with 3 base Might and Shield 2 is in combat as a Defender. Since Shield applies, its current Might is 5. A player chooses it as the target for Last Stand, a spell that reads in part “Double a friendly unit's Might this turn.” Its current Might is 5, so it gets +5 Might this turn, for a current Might of 10. After combat, Shield no longer applies, but the +5 Might from Last Stand does, so the unit’s Might is 8.
433.1.b.
Some Game Effects may also cause Damage to be Doubled; in this case, those effects are the equivalent to a replacement effect that reads “If I would take damage from a source, I take double that damage from that source instead.” or “If [I/this] would deal damage, [I/this] deal double that damage instead.” The source of the Damage in these cases remains the same after Doubling.
433.2.
Doubling is a Limited Action.
433.2.a.
Players may only Double when Game Effects direct them to do so.