Meatball grants Seiji the ability to manipulate raw human flesh.
The user can manipulate their own flesh, giving them the ability to separate their limbs and control them at will.
Flesh can be expanded and molded together for offensive and defensive maneuvers. It can be levitated and used as projectiles to capture targets.
Anyone captured by a ball of flesh can be kneaded and rolled into a meatball, rendering them immobile and disfigured. People who have been turned into balls of flesh still maintain all their senses. Seiji must maintain focus to keep their targets restricted, and this hold can be released if the user takes enough physical damage. 
Seiji has great control of his Quirk and mainly focuses on kneading his enemies into helpless meatballs. He fights from a distance, using flesh from his arms to create flesh bullets to barrage his opponents into submission.
Seiji has very delicate control over his severed flesh and understands it only takes one touch to defeat his opponents. He can manipulate his own flesh at a much greater degree than other people's, freely able to levitate several pieces of his flesh at a time, and can use them to transform several enemies at a time into meatballs.
While fighting from a distance, Seiji defends himself by rolling flesh together to create shields that can withstand Katsuki Bakugo's Armor Piercing Shots. He distracts his opponents with attacks while secretly ambushing them with hidden flesh balls from behind.