Amplify is a keyword ability
that allows you to boost the power and toughness of your creatures by revealing cards from your hand that share creature types with them.
Amplify is a static ability
which uses a replacement effect
to modify how the creature it's on enters the battlefield. As a permanent with amplify N is entering the battlefield, you may choose to reveal any number of cards from your hand that share at least one creature type with it. For each card revealed this way, that permanent enters the battlefield with N +1/+1 counters on it.
- If a creature with amplify has multiple creature types, you can reveal cards of any of its types.
- Since the creature enters the battlefield with the amplify counters already on it, there's no time to do things in between the creature entering the battlefield and getting the counters. By the time you know how many counters the creature will have and can do anything, it already has them.
- ...with itself: If two creatures with amplify are entering the battlefield simultaneously, you can reveal the same cards for both of them. (Assuming the types match up, of course.)
- ...with copy effects: If you use a Clone or a similar creature to copy an amplify creature, you reveal cards of the copied creature's types, not of the Clone-card's normal types.
Comprehensive Rules Entry
As of Return to Ravnica the full, official rules for this ability are as follows:
702.36a Amplify is a static ability. "Amplify N" means "As this object enters the battlefield, reveal any number of cards from your hand that share a creature type with it. This permanent enters the battlefield with N +1/+1 counters on it for each card revealed this way. You can't reveal this card or any other cards that are entering the battlefield at the same time as this card."
702.36b If a creature has multiple instances of amplify, each one works separately.