Power shield canceling (PSC) is an advanced technique in Super Smash Bros. Melee, Super Smash Bros. Brawl and Super Smash Bros. 4, which allows a player to cancel their shield drop animation with an attack after performing a power shield. This gives the player the ability to use any attack right after they power shield an attack, as opposed to having to wait for the shield drop animation to end (which is what the player has to do if they do not power shield an attack). While the technique was introduced in Melee, it was first discovered in Brawl.
How to perform
In order to power shield cancel, the player first has to power shield an attack (specifically, an attack which is not a projectile). Once they have power shielded an attack, they then need to let go of the shield button and wait for their character to drop their shield. The player can then perform any ground move. In Melee, the player can input an attack on frame 2 of a shield drop while in Brawl and Smash 4, they can input an attack on frame 1 of the shield drop however, a forward smash and down smash can still only be performed from frame 2 and they are more difficult to perform, as mistiming them will result in a roll or spot dodge.
While difficult to perform (due to the tight timing on the power shield and the varied amount of time a player can drop shield depending on the amount of shieldstun they suffer from), power shield canceling is a very useful technique as it can allow for:
As an example of this, if Zelda is hit by a move which is -5 on shield (meaning that she can act five frames before her opponent can) and she shields it normally, she has no way to punish the move, as all of her OoS options are too slow, so her opponent can avoid any punish she attempts. If she power shields the move however, she can perform a power shield cancelled down smash, which will come out quickly enough to punish the move before her opponent can act.
As a result, this is one of the main reasons why power shielding is so effective (along with it negating shield damage and having reduced shield pushback (as of Brawl)), as it allows for moves to be punished in situations where they would otherwise be safe.
Notably, Yoshi is incapable of power shield canceling in Melee and Brawl, which particularly crippled him in Brawl as even when he powershielded an attack, his fastest punish was still his frame 17 grab (due to his long shield drop animation and his inability to jump OoS) and unlike in Melee, he does not have reduced shieldstun or the ability to parry in order to compensate. This was fixed in Smash 4, as Yoshi gained the ability to power shield cancel much like any other character.
Differences between games
In Super Smash Bros. Melee, power shield canceling was easier to perform, as the game has a four frame power shield cancel window (against non-projectile attacks) and shield drop animations were 15 frames long across the board. This made power shield canceling highly effective in Melee as it meant that shield dropping after a power shield was a viable and effective option to punish attacks, which is not the case with regular shielding. A downside with power shielding however is that it has increased shield pushback, which means that power shield canceling may not always be useful, particularly against spaced moves.
In Super Smash Bros. Brawl, the technique became harder and less vital, as the power shield window was reduced to three frames and shield drop animations were reduced to seven frames. Despite this, power shield canceling is still a very useful technique and power shielding received an addition benefit, as it now removes almost all shield pushback. In conjunction with Brawl's reduced shieldstun, power shield canceling can be used to punish more attacks and it is now a much more effective way to to punish multi-hit attacks. It is notably a very useful technique to deal with Meta Knight's Mach Tornado, as the player can punish the move, as opposed to having to sit and shield and watch the move drain their shield down/shield poke them.
In Super Smash Bros. 4, the technique was initially extremely similar to Brawl, with the exception that the power shield window was increased from three frames to four. As of update 1.1.1 however, its window was reverted from four frames to three, making it more difficult to perform. In return, power shielding decreases shieldstun by 0.66×, compensating for the increased shieldstun in said update. This also means that power shielding now had lower shieldstun than regular shielding, giving power shielding opponents a greater frame advantage when shielding attacks, further increasing the reward of power shield canceling relative to shielding a move regularly.
With the lack of power shielding in Smash 64, power shield canceling was naturally not possible at all and it is no longer present in Super Smash Bros. Ultimate, due to the drastic change in how perfect shields function (although perfect shielding in Ultimate still allows the player to instantly use any of their attacks without going through the shield drop animation).