Perfect shielding (ジャストシールド Jasutoshīrudo, Just shield) (also known as powershielding and flash shielding) is the official term for a technique where one activates a shield just as 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 damage and may immediately perform a counterattack while the attacker is stuck in freeze frames. When used, any incoming attack is blocked with no damage done to a character's shield. The perfect shield technique in SSBB 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 two 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 almost never happens, and it merely redirects the attack at an angle instead of actually reflecting it to hurt the attacker.
In SSBM, 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 overlapping of the incoming attack. The noise heard by doing this is rather quiet compared to Brawl's, and the technique can reflect projectiles (which only do half damage).
Something still not completely understood is how Yoshi's shield sometimes powershields attacks even when the shield is already up.
The ability to powershield returns in SSBB, 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 slower gameplay. Also, powershielding rarely reflects projectiles, and even when it does, it merely redirects them back at an angle, not harming the attacker.