Limiter

No matter how much effort one puts in, every living being has an intrinsic limit to its growth. Too much power becomes unbearable and overwhelms its host, turning it into a mindless, rampaging monster. To ensure that we do not enter the realm where we lose all purpose and reason, God has set limits to the growth of every being. The mechanism with which growth is controlled is called the limiter. A limiter (リミッター, Rimittā) is a theoretical barrier that restricts the physical growth of a being. Genus first coined the term to explain how Saitama obtained his strength. According to his theory, God placed a growth limitation on every being to prevent them from losing purpose. Everyone eventually reaches this upper limit, where they become unable to develop more strength no matter how hard they try. However, it is possible for an individual to remove their limiter, which results in the development of immeasurable strength.

Psykos theorized the limiter independently of Dr. Genus, although she uses the term limit of growth (成長の限界, seichō no genkai) instead.

Removing the Limiter
In rare instances, it is possible for certain individuals to remove their limiters. Genus claims that this requires certain conditions. According to Genus, becoming a Mysterious Being is not the same as removing one's limiter: they are just reborn as a new being with a new limiter. Abnormally powerful humans, such as heroes, always had the potential to become extraordinarily strong, and those who modify their bodies must modify again in order to improve beyond their natural limitations; they also have not broken their limiters. The same applies to creatures that were born as monsters. They have natural physical capabilities far superior to ordinary humans. However, this only means that they have a greater limiter than humans; they did not remove their limiters.

Saitama was an ordinary person with no talent or potential but, through effort alone, removed his natural limits and achieved strength not measurable even by the genius Dr. Genus. However, his strength made him bald and left him with an overwhelming sense of alienation. As a person without a limiter, Saitama's strength continues to grow non-stop, even at an exponential rate upon experiencing intense emotions. Because he was already too overwhelmingly powerful, Saitama's growth went unnoticed by the others. However, after becoming influenced by "God", Garou managed to copy Saitama's power and realized that the hero is in fact still growing stronger. Undergoing an upsurge of emotion during his fight with Garou on Io, Saitama's growth rate became so high that Garou, who was repeatedly copying his strength and fighting back with equal blows, could still not keep up with him and was unable to register how strong the hero had become by the end.

Psykos's Method
I did everything I could to figure out a way to break through the limit of growth: monster cannibalism, blood and cell transfusion, breeding, experiencing different types of pain and stress, increasing their level of anger and hatred. Everything I could think of, I've tried. During this process, I found out that it is possible to reach the next stage if one conquers death. And if the process is repeated, you end up with explosive growth. Psykos seeks to create the "strongest monster" by breaking its limiter. She found that simply transforming humans into monsters gave them "no prospect of further growth" afterwards. After many failed experiments, she managed to find a way to seemingly overcome this limit: overload the body and spirit of a human by pushing him or her to the brink of death repeatedly, with the intensity tailored to the individual. Despite how simple it sounds, death can occur at any time in the process, and even if one survives, they might lose their minds, rendering the monster "mediocre." Although Psykos had made several monsters that grew fairly powerful this way, most of them eventually died before reaching Disaster Level Dragon. She does claim to have one success though: Orochi, the Monster King.

She attempted to repeat the process to Garou by sending a bloodthirsty Royal Ripper and Bug God to tail him, knowing that they will fight him to the brink of death. These conditions, alongside the many other near-death battles Garou has fought, allowed him to seemingly break his limiter and become an immensely powerful monster that continuously grows stronger, just like Orochi.

During Garou's battle with Superalloy Darkshine, there is narration stating that "Garou's limiter ... is starting to break!"

Phoenix Man, as a result of his unique ability to resurrect from the brink of death and being pushed to his limits by Child Emperor experienced an explosive growth in power and gained new abilities, lending credence towards Psykos' method. He cited his earlier conversation with Psykos about the Monster King and becoming stronger after conquering death as an explanation for his increase in power.

Garou also mentioned that his body felt light as if his "spirit" was removed from his body. He experienced explosive growth and quickly increased his speed, strength, and other abilities. Even while in the midst of battle, he could even adapt to psychic abilities. When he punched Golden Sperm, the latter remarked that Garou's punch was many levels higher than what humans are normally capable of. In the end, however, Garou began running out of fumes and lost the power he had gained. Whether this means that he had failed to completely remove his limiter, that his limiter was placed back on, or some other reason remains unclear.

Trivia

 * Different phrases have been used to describe removing the limiter: Dr. Genus says Saitama "removed his limiter" (リミッターを外す, Rimittā o hazusu) and "broke his limiter" (リミッターを破壊する, Rimittā o hakaisuru); Gyoro Gyoro says she wants to "break the limits of growth" (成長の限界を破る, Seichō no genkai o yaburu). Since they are all used in the same context (Genus even uses both "remove" and "break" in the same conversation), they are all presumably interchangeable.
 * In the webcomic, Psykos does not know about the limiter; Orochi does not exist in it either. She also had no direct involvement with Garou's limiter, unlike in the manga, where that is actually her intention.