The Mirror Shield (鏡の盾 Kagami no Tate, Shield of the Mirror) is Pit's down special move, which involves Pit taking out a large reflective shield that acts as a very strong defense against almost all attacks.
For as long as the user holds down the special move button (up to 15 seconds), Pit will crouch and hold the Mirror Shield in front of him. While the shield is active, it reflects projectiles. Reflected projectiles do 1.5x more damage, and will be accelerated to 1.7x speed, similar to how Wolf's Reflector works. Using the move as a reflector is faster than using the Angel Ring. Pit says "Nice try!" whenever he successfully reflects a projectile.
The shield can also block most non-projectile attacks. However, the attacker and Pit must be facing opposite directions for the attack to be blocked. As long as the two are facing each other the attack will be blocked should it hit the shield, even if only a small part of the hitbox connects with the shield. Likewise, if the two are facing the same direction, the shield will have no effect, regardless of how much of the attack hits it. The Mirror Shield can block any number of physical attacks without breaking and is useful against slow-to-start moves such as the Falcon Punch.
When initially brought out, the shield gives launch resistance and can reverse enemies when they attempt to attack Pit, like a combination of Mario's Cape and Counter. This can be used on moves such as the Reverse Falcon Punch and Warlock Punch to make the attacker get sent extreme distances.
Tilting the control stick/d-pad upwards while holding the special button will aim the shield at an upward angle (approximately 45°). This usually has no effect on the reflection, but interestingly, the shield can actually block most of Samus's Zero Laser Final Smash by angling the shield up at the correct distance; in this case, only the final hit will inflict any damage or knockback. However, the laser will still push Pit back before the last hit.
Pit's shield can be used to block Tabuu's Dragon Laser. Pit won't take damage but he will be pushed back. It can also block Tabuu's Pinpoint Explosion. Pit won't take damage, knockback and he won't be pushed back. It can also reflect Ridley's wind attack. This will damage the boss, and Pit will not be pushed back.
 Description from the Instruction Booklet
Projectiles that hit the Mirror Shield bounce back and cause more damage than they did before.
A glitch occurs when two Pits use their Mirror Shield to reflect certain items. Normally, after an item is reflected too many times, it causes a Shield break. When this glitch occurs, one Pit will either slide back a short distance, or will continuously reflect the item.
The Mirror Shield is one of the three Sacred Treasures Pit needed to defeat Medusa. It is based on Greek mythology, where a mirrored shield is used to defeat Medusa by looking at her reflection in it rather than her.
 External Links