Flak Jacket

Flak Jacket is a Call of Duty:World at War second tier perk that reduces explosive damage done to the player.

In Game
This perk allows a player to survive explosive weapons, such as the Bouncing Betty, excluding a direct hit from a Bazooka or a direct hit from a tank shell. This only applies to a single explosive, as a second explosive could do up to 70 damage, killing the player. It is canceled out by Fireworks.

This perk has no effect in Hardcore.

Tactics
This perk is commonly used in Headquarters and War to prevent being killed by incoming grenades while trying to capture a flag or HQ. It is also especially usefull in Search and Destroy, as the enemy will plant Bouncing Betties on the target as to kill the player planting the bomb.

Trivia
The flak jacket was probably the developer's response to grenade mortaring.