A shield is a bubble/egg encircling the user, that blocks all attacks with the exception of grabs and Final Smashes. One raises the shield by pressing L, R or Z Button. when standing on the ground.
History
The design for the shield probably originated from the SNES game Kirby Super Star. When Kirby possesed the "Mirror" ability, by holding "L" or "R", he could create a multi-colored ball around his body that would protect him from any attack other than grab attacks and even reflect some attacks. This shield was more effective than the "guard" command he normally had. (He would still block the attack, but certain powerful attacks would deal him minimal damage).
Technical details
Though the shield deflects most attacks without fail, it does not always cover the character's entire damageable collision bubble - thus, the user is still vulnerable to shield stabbing. Additionally, the shield can be shrunk by holding it for too long, or after being hit by enemy attacks. When the shield shrinks so much it disappears, the shield breaks and the character becomes dazed for a period of time, unable to act or attack. If a projectile hits the shield bubble at just the right angle, it will be deflected, rather than blocked.
The initial raising of the shield creates a small energy wave; if that wave hits a projectile, it will be reflected back.
If using the L or R button to shield, one can adjust the depression of the button to alter the density of the shield. When held very lightly, the shield is large and shrinks slowly, but if held all the way down, the shield is small and shrinks quickly. The denser the shield is, the less knockback occurs when the shield blocks an attack. If one uses the Z button to Shield, then the shield will be a maximum size, however this shield will take a bit longer to put up as the character will try to grab, first.
Interestingly, variable shield density has been removed from Brawl. Density was presumably removed because the Shield button can be assigned to buttons that aren't as pressure sensitive as the Gamecube Controller's L and R buttons.
Options from the shield
When holding their shield, characters do not have all the same options as they do when standing, but they are still able to use many moves. A character can roll to reposition themself while retaining invincibility. If a player predicts that the enemy will grab to penetrate their shield, they can quickly sidestep to avoid the grab. If the control stick is tilted slightly, the shield can be moved slightly to help block attacks from specific directions if the shield gets too small.
Characters can also grab and jump out of their shield. Grabbing after blocking an attack is a prevalent technique known as shield grabbing - it allows one to quickly retaliate against an enemy attacker. Jumping from the shield allows characters to immediately shffl, wavedash or jump-cancel into their up smash or up B attacks. All these offensive and defensive options make shielding the centerpiece to a player's defensive game.
Yoshi's shield
Yoshi has a different shield from that of other characters'. Instead of using a bubble, he uses an egg for protection. This egg does not shrink, preventing any attempts at shield stabbing. As it weakens, the egg darkens from white to dark red (similar to the colour of the percentage meters). The egg prevents Yoshi from jumping out of his shield, but when lightshielding, Yoshi's traction is reduced to less than Luigi's, and will slide a huge distance when attacked. Also, Yoshi has been known to randomly Perfect Shield (not completely understood), and is also completely invincible during his shielding animation.
Jigglypuff's Shield
While having a very large shield compared to Jigglypuff's size, if Jigglypuff's shield breaks, it flies to the top of the screen and, if nothing stops it, Jigglypuff will self-destruct. This phenomenon is known as the Shield Jump and can be helpful sometimes.
Notable Shields
- Yoshi's Shield uses an egg instead of a bubble. (see above)
- Jigglypuff's Shield is very large, but if it breaks, she covers a massive distance enough to KO.
- Donkey Kong's Shield is known to be very small related with his size (though it's big), making him very easy to shield stab.
- Pikachu is known for having the smallest shield size, making both easy shield stabbing and shield breaking.