Dr. Genus’s experiments involved heavy shock therapy as well as the injection of an experimental serum (guess what color it was). At first it took nearly killing the boy to coax out any sort of response, but over time, a mix of the serum’s improvement and Genos’s mental and emotional deterioration altered the inner barrier that keeps a human from becoming a monster. And so it became easier to draw out the monster-like state.

Genos's senses become enhanced but his rationale becomes heavily impaired. His mind returns to the lab, and he is filled with the constant urge to destroy things, and damage anyone that comes close until he exhausts himself.

Once he snaps, he has maybe a handful of seconds of clarity before his body changes, electric-like power coursing through his body and keeping him toasty-warm to the touch. He becomes extremely durable, heals very quickly, and gains heavy physical strength. That would be the middle stage in the pic above, and he’ll usually stay at that stage unless his life is seriously threatened (holding him down doesn’t count, it has to be a violent threat).

All his enhancements get way worse if he gets any angrier, and by the time the sparks start, S-Class heroes would have a hard time taking him down, despite his compact size. He’d go from “destroy stuff and kill anything that bothers you” to “just fuck everyone’s shit up and don’t stop until nothing’s breathing.”