A perfect shield, also referred to as power shielding, is both an effect and a technique in Super Smash Flash 2. Because a normal shield is a bubble of energy that starts to dwindle in size and weaken the instant it starts, it is the strongest at the very first frame the shield is activated. This first frame of the shield is the "perfect shield".
When a hitbox (disjointed, melee or otherwise) hits a perfect shield the entire screen will flash white for a brief moment, followed by a pinging sound.
Perfect shields prevent a shielding character from being knocked backwards, and the attack that was perfectly blocked will not cause damage to the shield. If the hitbox in question is a projectile, a perfect shield can reflect it back at the attacker like in Super Smash Bros. Melee. Projectiles are sometimes reflected much quicker than the projectile's normal speed. (Goku's Ki Blasts, when perfect shielded, will reflect with more than three times the projectile's normal speed. The same applies for Naruto's Shadow Clone Toss as well as several unnamed others.) However, some projectiles can not be reflected at all - even by a perfect shield. Instead, they will continue along their normal path but will be able to hit their original owners.