Most creature based graveyard mechanics (like Unearth, for example) tend to revolve around somehow getting that creature back on the battlefield, but scavenge is different. It appears only on creatures, and can only be activated when that card is in the graveyard. At sorcery speed (on your turn in your main phase with nothing else on the stack), you may pay the listed ability cost of a scavenge card in your graveyard and exile that card. You then put a number of +1/+1 counters on a creature you control equal to the power of the exiled card.
So yeah, you’re eating a creature from the graveyard to make another creature bigger. There’s really not a lot else to it. Exiling the card from your graveyard is part of the cost, so once scavenge has been activated, it’s too late for your opponent to try to get the card out of your graveyard to stop the scavenge. Scavenge does happen at sorcery speed though, so if an opponent does try to exile the card from your graveyard (or otherwise remove it from there) you can’t activate the ability in response. They can still remove the creature that scavenge is targeting, however. If they do, the effect fizzles, and the creature is still exiled from your graveyard, so choose your spots with care.
Being pretty simple and buffing up creatures, scavenge was decently well received when it came out. People like getting value out of there creatures in the graveyard, and it’s not at all a recursive mechanic since you aren’t returning the creature somehow. Don’t be surprised if it makes a comeback in a Standard set sometime in the future!
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