Mahito first developed this technique while he was close to death during the climax of his battle with Yuji and Nanami. Using death as an inspiration, Mahito was able to increase his power and unlock the ability to utilize a Domain Expansion.
Mahito's Domain Expansion is conjured by hand seals formed by tiny hands inside Mahito's mouth. This creates a large black environment that presents Mahito's target with giant hands clasped together to form a flower-like shape. Connected to these hands are multiple arms that grabbing one another to form a net pattern that surrounds Mahito's target.
Like all Domain Expansions, Self-Embodiment of Perfection increases the effectiveness of Mahito's Idle Transfiguration Cursed Technique and makes sure it hits the soul of the target every time. Nanami described the experience of being inside Mahito's Domain as literally feeling as if he's in the palm of Mahito's hand.
However, a major drawback of this ability is that Mahito is forced to touch the Soul of anyone inside. This backfired when Yuji entered the Domain from the outside and Mahito touched Sukuna's soul a second time, prompting the King of Curses to lash out at Mahito.