Idle Transfiguration allows the user to reshape the soul of themselves or anyone they physically touch. The shape of the body is dependent on the shape of the soul, so if the soul is distorted, so too is the body. Therefore, someone using Idle Transfiguration can disfigure their target's body into something grotesque by mutating their soul with just a touch. 
This Cursed Technique attacks the soul directly. Only under certain circumstances can someone protect their own soul and anyone who cannot is instantly killed or deformed by Idle Transfiguration. All ordinary people eventually die after having their soul transfigured.
The user can also reform the shape of their own soul and heal any injuries their body sustained in battle. This renders traditional means of injuring the user useless, as their soul must be attacked to do any significant damage. The user can even transform their body to give them an advantage, such as giving themselves wings for flight or shrinking their body to avoid an attack.
An individual facing the threat of Idle Transfiguration can stay out of range and use Cursed Energy to protect their soul from being distorted. However, Idle Transfiguration can still injure the body if the user touches their target, even if it doesn't fully distort their soul enough to deal a fatal blow. Even so, after two or three more direct to attacks to the soul, even if it's being protected by Cursed Energy, the target will no longer be human. 
In his spare time, Mahito uses Idle Transfiguration to experiment with human souls. For example, he mutates human bodies to see how big and small they can get via disfiguration. He even carries small mutated ex-humans on his person, which can be manipulated further to create larger monsters that obey Mahito.
Mahito will often mutate humans by placing his hand on them without killing intent and then suddenly disfiguring their body. Additionally, he uses disfigured humans as monsters to attack his enemies.
In battle against Jujutsu Sorcerers, Mahito mainly aims to touch their soul and mutate their body like any other human. He has stated that strong Jujutsu Sorcerers like Kento Nanami are perfect for experimentation.
In the rare situation that he's unable to touch his opponent's soul (such is the case with Yuji), Mahito will rely on his personal monsters or changing his body into a variety of different shapes for offensive and defensive purposes.
By reshaping his own soul Mahito changes appendages body into animal limbs for greater mobility, into weapons for increasing killing power, and many more potential abilities. He also uses this same application in order to heal himself after sustaining an injury to his body, and can even heal humans if he so chooses.