In Call of Duty: World at War, it decreases the bullet damage the player takes by 25%. The Flak Jacket perk was created to be the antithesis to explosive weapons, making Juggernaut less powerful in Call of Duty: World at War. In almost all situations, Juggernaut will allow the user to survive one more shot. A Juggernaut symbol (a plus sign in a red and white circle) will appear just below the cross-hairs when a player hits an enemy player with the Juggernaut perk.
The perk symbol in the Wii version for Call of Duty: World at War is different from other console versions, it is a shield instead of a cross with a bullet over it. This is perhaps due to the symbol's resemblance to the Wii's D-pad, which caused it to be changed to avoid confusing the players.