Perfect shield
Perfect Shield is the official term when one uses a shield just in time. There is no "exact" way to perform the Perfect Shield technique. 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 may take no damage and have a proper moment to perform a counter attack while the attacker is still recovering.
When used, any incoming attack is blocked with no damage done to a character's shield, and projectiles may also be reflected. Because powershielding is technically a shield, it is ineffective against grabs.
Powershielding is done by quickly and fully depressing L or R so that the incoming attack is inside the character's shield. If done correctly, there will be a white flash and a distinctive (though somewhat hard to hear) sound. If the timing is slightly off, then the attack may be blocked by a lightshield, a normal shield, or it may not be blocked at all.
CPU players, especially level 9s, use powershielding a lot. Human players don't use it as much simply because of the reaction times and precision usually required. As such, powershielding often comes as a surprise and can momentarily disrupt a match.
The name Powershield comes from a bonus awarded for performing several in a row. A recent Dojo update has given this technique the name "Perfect Shield". [1]
Something not completely understood is how Yoshi's egg shield sometimes powershields attacks even when the shield is already up.
In SSBM, the powershield can reflect projectiles. There is a distinctive noise when this happens and the character's shield flashes. However, unlike the Reflector, the damage when reflecting projectiles is reduced by half.
