Through his Quirk, Chisaki is able to disassemble and then reassemble anything he makes physical contact with to its original state or a completely different one.
With it, Chisaki is capable of instantly killing people or destroying body parts, ripping body parts away and fusing them to foreign objects, curing all of a person's chronic ailments, and likely much more. While he is very capable of using his Quirk to kill, he is also capable of using it to "revive" people, as he has done to Rappa multiple times. Because of this, Chisaki can afford to be reckless with his power, as any collateral damage can just be reversed back to an undamaged state if necessary.
Additionally, it's also effective at covering Chisaki's escape tracks - His operating base has multiple corridors with dead ends, which he can use his Quirk on to disassemble and then reassemble once he has passed through. He is also capable of using his Quirk on his surroundings, such as the ground, and then rearrange it into a most favourable terrain for himself, and hostile for his enemies.
Chisaki can also use Overhaul on himself as an inward version of his power. By disassembling himself and another person, and then reconstructing both bodies at the same time, Overhaul can combine elements of the other body into his own. In that state, he becomes a monstrous version of himself, increasing his already great strength and speed. Chisaki can use this same ability on other people and objects as well.
Chisaki's main usage of his Quirk outside of battle is obtaining DNA samples from Eri, which is done by destroying her body, collecting some of her remains and reassembling her back to life. Eri's blood contains Quirk-destroying properties, a result of her own Quirk, making her valuable to Chisaki.
As Chisaki wears surgical gloves all the time, he needs to remove them first before attempting to use his Quirk, although if angered enough, he can just destroy them directly. Also, as the destructive portion of Overhaul often results in high amounts of gore when used on people, it may trigger Chisaki's mysophobia if the blood splatters on his body.