The user quickly swipes upward in a high-spiraling leap, with their blade extended horizontally-to-vertically. It provides decent vertical recovery, but very little horizontal recovery. It can be used as an out of shield option and also to edgeguard an opponent.
Dolphin Slash's damage and knockback is strongest in its first few frames of attack. The longer the attack is out, the weaker it gets. It's best used point blank if it's meant to be used as an attack, since the later frames of the attack are weaker than the beginning frames. It can be slightly angled forward, similarly to Roy's Blazer.
Reverse Dolphin Slash
A "Reverse Dolphin Slash" can be performed by starting the attack and quickly moving the joystick backwards. This will cause Marth to turn around just before jumping upwards, and if it hits an enemy close to him as he starts the move, it will almost guarantee that Marth hits with the sweetspot, due to the hitbox placement of the attack. Due to these properties, the Reverse Dolphin Slash can act as both a powerful surprise KO move as well an effective move to gimp offstage. Often Marth players will use this move as an alternative finish to a Ken Combo should they be too close to sweetspot Marth's down air spike. Additionally, due to its speed, some players will opt to use the move to punish poor tech rolls.
In Brawl, the regular Dolphin Slash appears to be at least as powerful as Melee's Reverse Dolphin Slash. Marth can be seen jump-cancelling a shield into it for potential kills at about 100% (130% on heavyweight characters).
This move in Brawl has several more advantages than the version in Melee. It has more horizontal range, can be angled more noticeably forward, and its vertical recovery is still great. It is also one of the few moves to retain the speed it had in Melee. However, it lacks the higher potential to KO in comparison.
What makes this move really useful are the invincibility frames starting on frame 1 to 5, allowing Marth to escape chain grabs and multi hit attacks. However, when used for this purpose, Marth is left helpless and vulnerable to attack. The move hits the opponent on frame 5, like it did in Melee.
Dolphin Slash returns in Super Smash Bros. 4 as Marth's up special move, and his clone Lucina also possesses the move as well. Its sweetspot is very slightly smaller, making it a bit harder to hit a reverse Dolphin Slash. However, all other hitboxes of the move are slightly larger. It has significantly less landing lag than in previous games, making it safer if the user is forced to land helpless. However, the grounded version now only has two frames of intangibility (frames 4–5), making it more unsafe as an out-of-shield move against lingering hitboxes. Before update 1.0.8, it dealt drastically reduced knockback, making it useless for KOing until extremely high percentages where it would not be viable.
However, starting from the 1.0.8 update, Dolphin Slash's knockback has been significantly buffed. It now has KO potential at percentages around 130% (significantly lower with rage), and due to its speed it can easily catch opponents off guard, along with giving the user a hard-hitting option to escape pressure if platforms are available. The landing lag was also reduced further in the same update. As such, it is considered a much more viable move than in previous patches, and arguably gives back the strength it had originally lost from Brawl.
Although hitting a reverse Dolphin Slash is now very rare, the move is still very useful for edgeguarding and stage spiking due to its speed, especially against poor and slow recoveries such as Dark Dive and Duck Hunt Jump. Dolphin Slash is now arguably a better recovery move with the removal of edge-hogging, as the move still has great vertical reach and speed. It still has limited horizontal movement, but can still be angled slightly.
Dolphin Slash remains the same in Ultimate but has slightly more landing lag.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Although the move has not been named or seen in any of the Fire Emblem games, Dolphin Slash's animation loosely resembles the jumping portion of the critical hit animations of the swordmaster class from Fire Emblem Thracia 776. This follows the trend of Fire Emblem's unique animations being given a name and ability in Super Smash Bros., such as Shield Breaker being based on Marth's standard attack animation.
The name could be a reference to the GameCube itself, as it was codenamed the Dolphin. It may also reference Marth's royalty, as the French word dauphin—which directly translates to "dolphin"—is defined as "the eldest son of the King of France." Marth is a firstborn prince, and therefore the term dauphin could be applied to him.