When the move is initiated, Ness enters a hunched-over pose to prepare the attack. This causes a ball of green light to appear above him like so. As the special move button is held, the ball of light moves downwards and increases in power, until the special button is released or until it is fully charged, at which point it explodes.
PK Flash can be charged until it falls to the altitude from where Ness initiated the attack. It can be slightly moved horizontally and vertically by using the control stick. In Melee, Brawl, and Smash 4, if the attack falls upon a platform or a higher level of a stage before it is detonated, it will disappear harmlessly. This can be a problem, especially in Brawl, since it falls below Ness' height when fully charged (but nonetheless, it is still considered fully charged if the player lets go of the special button right before it hits the ground). The ball of light is easier to control in Brawl than it is in Melee. In Ultimate - it can cover more vertical distance than all other games, but it has slightly less kill power.
In all games except Ultimate, when performed in the air, it puts Ness into a helpless falling animation. Fully charged, without the influence of special conditions, it deals a maximum damage of 39% with huge knockback.
Due to Ness's accent, many players mishear "PK Flash" as any of a variety of phrases, such as "PK Pulse" and "PK Cross", as the final "sh" sound is quieter than the rest of the yell. However, when Kirby copies Ness, he clearly says "PK Flash" when he uses it, despite both characters having the same voice actress. In Super Smash Bros 4, due to Ness's new voice clips, PK Flash is now heard correctly.
Of all projectiles, PK Flash has the highest knockback when fully or almost fully charged, and KOs as low as 60%. In SSB4, PK Flash covers more range behind Ness, whereas in previous games, PK Flash has reduced range if shifted behind.
Originally, in Melee, the attack is colored green, while in Brawl and SSB4, it is given more of a blueish tint.
Due to the move's lag and the fact that it leaves Ness vulnerable, some players try to avoid using this attack. However, it does have some uses. For example, it can be used for low percentage Star KOs when used on an opponent that is landing. It can also be used for edgeguarding, attacking recovering opponents and/or gimping their recoveries.
The move also has good uses in Team Battles. It can be absorbed three times (or once in SSB4) by Mr. Game and Watch's Oil Panic, giving him a OHKO. It can also be used for healing a Lucas, another Ness or a Mii Gunner by having them use PSI Magnet or Absorbing Vortex, on it. Finally, Ness' teammate can grab his opponent, allowing Ness to use his fully charged PK Flash.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
In Brawl, if PK Flash is spammed when Ness is directly on top of a orange or green spring within a custom stage, he can stall inside of the spring for short periods of time. After a short amount of time, however, he will bounce out of the spring as normal. This also works with PK Thunder.
PK Flash (localized as PSI Flash in EarthBound) first appeared in EarthBound and later appeared in Mother 3. In both games, the move involved using the character's psychic power to cause a strong flash of light that could affect all enemies. In EarthBound, Ness learns PK Flash. There are four levels to this power, which are: α (Alpha), β (Beta), γ (Gamma), and Ω (Omega). PK Flash α will only cause enemies to cry or feel strange. PK Flash β can also cause enemies to feel numb or be instantly destroyed. PK Flash γ and Ω each increase the chance of more powerful effects and decrease the chance of weaker ones. When Ness uses PK Flash in Smash, the player can either let PK Flash fully charge and "explode" or stop charging early for a less powerful explosion. PK Flash being weaker when less charged and most powerful when fully charged is likely a reference to the different levels of PK Flash in EarthBound, considering there are four different audio cues the detinating portion of the move can make depending on the charge starting in Smash 4.
In Mother 3, Lucas would learn PK Flash after he was shocked by an electrical discharge. The PK Flash in Mother 3 didn't have any levels, so PK Flash could deal minor status afflictions like crying or one-hit KOs at any point of the game.
The high damage and knockback this attack possesses in the Smash series is most likely a reference to the instant defeat chance it has in its home series.