The following contains spoilers for My Hero Academia Chapter 358, "The Guy Who's Made Some Progress" by Horikoshi Kohei, Caleb Cook and John Hunt, available now in English from Viz Media.
Before the latest chapter of My Hero Academia, Shigaraki Tomura had only offered a fleeting glimpse of what his Simple Growth abilities were capable of, yet it was enough for the heroes to decide he had reached an unprecedented threat level. He's easily holding back a team of more than six Pro Heroes, and felt comfortable enough to revel in his new abilities while pondering his plans for world domination. Here's how Shigaraki's Simple Growth works.
In line with My Hero Academia's established descriptive ability names, Simple Growth is exactly what its name suggests. Shigaraki is now capable of altering his body's composition at will, increasing or decreasing his overall mass without the aid of a Quirk or any support items. The fact that it's an ability innate to his body suggests the normal rules of Quirks won't apply to Simple Growth. It probably won't have a time limit attached to how long he can modify his body, while attacks against Quirks, such as Eraserhead and Monoma Neito's combined Erasure, will not affect it at all.
Simple Growth was the result of the transformative process Dr. Garaki put Shigaraki through to grant him the power he previously sought, and the All For One Quirk completely settling and taking root inside him. His body toughness was increased almost to the level of All Might in his prime in order to deal with the strain of dozens of Quirks, but Simple Growth was how he conquered the Quirk Singularity.
Every one of the Quirks Shigaraki inherited has different modes of activation and body parts they affect when in use. With Simple Growth's ability to manifest from body parts at will, Shigaraki will be able to use many different Quirks simultaneously without combining them into a single attack or over-burdening his conventional body.
The inner workings of Simple Growth's abilities finally being revealed has done little to clear up the confusion on whether All For One or Shigaraki himself is in control of the body. There doesn't seem to be a struggle for control anymore as Shigaraki is only focusing on one goal at the moment. He believes the power Simple Growth affords him to be absolute and that it'll only be a matter of time before the heroes facing him in battle fall -- and the society they want to protect shortly after.
Shigaraki further expanded on his vision for the world once it was conquered, explaining that the current Quirk society was split unevenly. He claimed it unfairly favored those with desirable Quirks that weren't off-putting visually. In what could only be described as a joke, he asked why the heroes who claimed to fight desperately for peace weren't in favor of his idea of complete submission to him. There would no longer be any caste systems based on the viability of Quirks as everyone would be forced to submit to him.
These lofty speeches in My Hero Academia are more characteristic of All For One than they are of Shigaraki. In his last unambiguous appearance, Shigaraki's goal was still complete societal destruction, with no ideas of reform or domination. All For One seems to have the upper hand over Shigaraki in the fight for his body as it stands -- but it's unlikely that this balance will remain unchallenged through the story's end.