"5-power matters" is the theme of the Shards of Alara's Naya shard; the Naya shard has a number of cards that look for creatures with power 5 or greater. These cards work in a variety of ways. "Power 5 or Greater" was featured as rules card 5 of 5 in the Shards of Alara set.
- If an ability says "target creature with power 5 or greater," it checks that creature's power twice: when the creature becomes the target, and when the ability resolves. Once the ability resolves, it will continue to apply to the affected creature no matter what its power may become later in the turn. (Relevant cards: Bloodthorn Taunter, Godtoucher, Mosstodon, Rakeclaw Gargantuan, and Spearbreaker Behemoth.)
- If an ability affects a number of creatures with power 5 or greater but doesn't target them, it checks the creatures' powers only once: when the ability resolves. Once the ability resolves, it will continue to apply to the affected creatures no matter what their powers may become later in the turn. (Relevant card: Gustrider Exuberant.)
- If an ability triggers when a creature with power 5 or greater comes into play, it checks that creature's power only once: when that creature comes into play. The trigger checks a creature's initial power upon being put in play, so it will take into account counters that it comes into play with and static abilities that may give it a continuous power boost once it's in play (such as the one on Glorious Anthem). After the creature is already in play, boosting its power with a spell (such as Giant Growth), activated ability, or triggered ability won't allow this ability to trigger; it's too late by then. Once the ability triggers, it will resolve no matter what the creature's power may become while the ability is on the stack. (Relevant cards: Godtracker of Jund, Mighty Emergence and Where Ancients Tread.)
- Some cards have abilities that say "At the end of your turn, if you control a creature with power 5 or greater, [effect]." These abilities have "intervening 'if' clauses." That means (1) the ability won't trigger at all unless you control a creature with power 5 or greater as your end step begins, and (2) the ability will do nothing if you don't control a creature with power 5 or greater by the time it resolves. (It doesn't have to be the same creature as the one that allowed the ability to trigger.) Power-boosting effects that last "until end of turn" will still be in effect when this kind of ability triggers and resolves. An ability like this will trigger a maximum of once per turn, no matter how many applicable creatures you control. (Relevant cards: Drumhunter, Exuberant Firestoker, and Sunseed Nurturer.)
- Some cards check the power of creature cards that aren't in play. If a creature card you look at this way has "*" in its power, the ability that defines its power works in all zones. (Relevant cards: Mayael the Anima and Sacellum Godspeaker.)
- Ferocious, a similar mechanic from Khans of Tarkir