PK Flash (ＰＫフラッシュ, PK Furasshu) is Ness' neutral special move in both Melee and Brawl. It is a chargeable attack. When the move is initiated, a ball of green light appears above Ness. As the special 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. The ball of light is easier to control in Brawl than it is in Melee. 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. 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). 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.
Of all projectiles found in Melee and Brawl, PK Flash has the highest knockback when fully or almost fully charged, and KOs as low as 60%.
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. The move also has good uses in Team Battles. It can be absorbed three times by Mr. Game and Watch's Oil Panic, giving him a OHKO. It can also be used for healing a Lucas or another Ness by having them use PSI Magnet on it. Finally, Ness' teammate can grab his opponent, allowing Ness to use his fully charged PK Flash.
PK Flash (PSI Flash in North America) 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. 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 and β will often cause enemies to cry, feel strange, numb, and in some circumstances, be instantly destroyed. PK Flash γ and Ω will often destroy enemies. In Melee and Brawl, Ness can use PK Flash. When he uses PK Flash, 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. 
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.