Passive shield
While all characters have access to a bubble shield, Link and his counterparts carry their own personal shields while in battle. While standing still, incoming projectiles that hit the shield will deal no damage and simply push Link back slightly. This effect was implemented in Melee and carries over into Brawl.
Since projectiles must hit the front of the shield to be blocked, Link may still be hit in the back, head, or feet, and parts of his standing animation may leave the shield temporarily useless. Crouching can migitate this somewhat, but then any projectiles that would have hit the shield normally will instead hit Link in the face. Link's shield is unusable when he is holding an item and Link's idle poses also prevent the shield's effects.
To be considered a projectile for the purposes of the shield, a hitbox must be both reflectable and blockable.
The fact that Link must be facing the projectile in order for the shield to block it is reminiscent of how Link used his shield in the NES Zelda games.
The design of each Link's shield is taken from the respective game. Link carries the Hylian Shield from Ocarina of Time in both Super Smash Bros. and Super Smash Bros. Melee while he uses the shield's Twilight Princess design in Super Smash Bros. Brawl. Young Link uses the Deku Shield from Ocarina of Time (which does not burn like its original counterpart), while Toon Link holds the Hero's Shield from The Wind Waker.
