Flanking

Flanking is a keyword ability that gives creatures that block a creature with flanking -1/-1, unless the blocker also has flanking. The only tournament-legal cards with this keyword were printed in the Mirage Block. There are twenty-one cards that have flanking or can grant it to a creature. The ability is found in all colors, but is primarily printed on white cards (38 %).

From the Comprehensive Rules

• 502.3. Flanking
  • 502.3a Flanking is a triggered ability that triggers during the declare blockers step. (See rule 309, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
  • 502.3b If a creature has multiple instances of flanking, each triggers separately.

As this is done during the Declare Blockers Step, if a creature is put into the graveyard as a result of the -1/-1, that creature will deal no damage to the creature with Flanking, even if it has first strike. The creature with flanking is still considered blocked, however, so no damage will be dealt to defending player, unless the creature with flanking also has trample.

Flanking only triggers when a creature with flanking becomes blocked, not when it blocks.