Flanking
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Flanking, at first glance, is like dealing 1 first-strike damage and having 1 extra toughness on the attack. This is not exactly true, though. When blocking a flanker, a nonflanking defender gets -1/-1 before damage is dealt. Reducing a creature’s toughness is much stronger than dealing first-strike damage, since healing and regeneration effects don’t work against toughness reduction.
For example, if Drudge Skeletons blocks a Black Knight, the Black Knight will destroy the Drudge Skeletons with its first-strike damage, but the Drudge Skeletons can (and probably will) regenerate. If Drudge Skeletons blocks a Zhalfirin Knight (2/2, flanking), the Drudge Skeletons will become 0/0 after blocking is declared. Regenerate won’t do a thing.
Also, the -1/-1 penalty applies to all blockers. Put Giant Strength on a Burning Shield Askari. Should your opponent block the enchanted knight, she would need to commit either a 5/5 creature or two 3/3 creatures to destroy it, and she would lose her blockers in the process. (Both 3/3 creatures are reduced to 2/2 when blocking the flanker.) A Giant Strengthed Burning Shield Askari attacking on turn 4 is not uncommon; defending with two 3/3 creatures on turn 4 is.