When performed, Ness uses his PSI powers to summon a barrage of psychic comets that rain from the sky, which can do 20% damage each and strong knockback if they connect with opponents. The comets fly down in a fan shape, similar to in Super Smash Bros. Brawl.
The origin of PK Starstorm comes from the game EarthBound, known in Japan as Mother 2, where it is actually learned by Ness's companion Poo, a prince from Dalaam and not Ness himself. However, in the description of the PK Starstorm for Ness in Brawl, it is hinted Poo teaches Ness this attack.
PK Starstorm's early design.
Ness unleashing the attack. He is in this position for the duration of the Final Smash.