Rule 432.1.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.