Perfect shielding (ジャストシールド Jasutoshīrudo, Just shield) (also known as power shielding, its official and common name in Melee) is the official term for a technique where one activates a full shield such that it overlaps with an incoming attack would have hit the receiving character. In order for the technique to be executed, one must rely on timing and skill. When the opponent is ready to strike, the player must quickly use the shield. If done correctly, the character takes no shield damage or shield stun and may immediately perform a counterattack while the attacker is stuck in hitlag. The perfect shield technique in Melee and Brawl is comparable to a parry in traditional fighting games, as it results in little to no knockback and freezes the opponent for a couple frames.
Perfect shielding is done by quickly and fully depressing a shield button four frames before an attack connects. If done correctly, there will be a significant flash on the shield and a distinctive "chlink" sound. Because it is technically a shield, it is ineffective against grabs.
CPU players, especially level 9s, use this technique a lot. Human players don't use it as much simply because of the reaction times and precision usually required. As such, it often comes as a surprise and can momentarily disrupt a match. Perfect shielding is often used to block a wave of incoming projectiles fired by camping characters, such as Falco.
In Melee, perfect shielding can reflect projectiles. In Brawl, this does not happen, and it merely redirects the attack at an angle instead of actually reflecting it to hurt the attacker.
In Melee, the technique is known as power shielding (named for the Power Shielder bonus attained by using the technique multiple times). Power shielding is accomplished by fully pressing L or R such that the shield appears just before the incoming attack would hit you. More specifically, the full shield bubble's outer rim must connect with the hitbox of the incoming attack within 4 frames of activating the shield. The noise heard by doing this is rather quiet compared to Brawl's, and should a projectile hit on frames 1 or 2 of the powershield, the technique will reflect projectiles at half the original damage (the knockback remains the same however). Though the technique is difficult to perform consistently, powershielding is extremely valuable in competitive play as it allows players to punish moves with low ending lag such as Fox's neutral aerial. Its ability to reflect projectiles is even more valuable as it allows players to punish otherwise safe, predictable projectile spam, particularly in the Falco matchup, whose lasers can shut off many options and leave a player completely helpless if not dealt with properly.
Though extremely difficult, powershielding projectiles is still humanly possible to perform consistently. Players who use characters with projectiles often have an easily trackable firing pattern, such as firing projectiles only when there is a certain distance between the players. It is also important to note. that players (again Falco mains in particular) will short hop to move while firing. As such, a projectile user running or jumping away from pressure is often a surefire sign he or she intends to fire. If the projectile is thin like Falco's laser, another thing a player can do to ease the risk and timing needed to powershield is to crouch and wait for the projectile to pass over the character. Shielding while the projectile is directly over the character will ensure a powershielded projectile.
Yoshi's shield sometimes powershields attacks even when the shield is already up.
 Chronological frame data
By pressing L or R during an actionable grounded frame, the Guard animation will be triggered. The ability to powershield with it depends on whether the button press was digital or analog:
1. Digital: GuardReflect is triggered. Reflecting projectiles is possible during the first two frames. During the first four frames, physical attacks will enable the subsequent cancellation of the GuardOff animation. Both is only possible if the incoming hitbox also overlaps the inner powershield sphere on the first frame of its collision with the shield:
2. Analog: GuardOn is triggered. This animation has no powershield sphere. It can be transitioned to GuardReflect by a digital shoulder button press only on frame one and only if no hitbox collided with the shield on this first frame.
 Example of multi-hit attacks that still preserve the physical powershield benefit
Fox stands in front of Marth and does a SHFFL down aerial:
The ability to powershield returns in Brawl, working the same way as in Melee. However, this time, a clear "clink" sound occurs when executed properly. It is also easier to execute than in Melee, because of the ability to buffer . Also, powershielding no longer reflects projectiles; should a projectile connect during powershield frames, they will rebound off the shield at an angle instead of rebounding back towards the owner of the projectile.