This Domain Expansion brings the user and their targets inside the Limitless itself, a vast void of infinite knowledge. Boundless raw information floods into the target's mind, overwhelming them to the point where they're completely immobilized.
The complete incapacitation of the target is caused by a phenomenon where the target will be able to see and feel everything while not being able to see or feel anything at the same time. All the actions one has taken in their life is forced upon them infinitely and they can't do anything but die slowly. This effect does not affect the user or anyone they're touching with no intent to harm.
Unlimited Void is one of, if not the most advanced technique in the arsenal of the Limitless. Satoru Gojo inherited the Six Eyes and likely mastered his Domain Expansion after awakening his full potential after the Star Plasma Vessel escort mission. He first displayed his mastery over his domain when he used it to counter Jogo's Coffin of the Iron Mountain. Unlimited Void was the more refined of the two domains and easily supressed Coffin of the Iron Mountain.
Being trapped inside the Limitless left Jogo in a complete trance, to the point where he couldn't move at all and could hardly think clearly. He could only think of how he was helpless due to the endless information invading his mind. Satoru divulged how Unlimited Void forces one's actions upon them infinitely and noted how it was ironic how the target is given everything and yet unable to do anything at all.
When Kenjaku was explaining his plan to seal Satoru with Jogo, a part of their strategy to use non-sorcerers to hinder his techniques was meant to prevent Satoru from casting Unlimited Void at all. They assumed Satoru is skilled enough to exclude anyone he's not targetting from his domain. However, doing so during the Shibuya Incident could've crushed the bystanders between the domain's barrier and the curtain trapping Satoru. Kenjaku was 99% sure Satoru wouldn't cast his domain and reminded Jogo that no one on their side could cast theirs either as that would leave Satoru no choice.
During their actual fight with Satoru, Mahito and Jogo were confident their adversary wouldn't sacrifice the bystanders' lives to cast his domain. Although Satoru could have exorcised every curse with Unlimited Void,, everyone else would've died too. Instead, Satoru surprised everyone and activated an all-or-nothing Domain Expansion of just 0.2 seconds. The time frame was just a guess that Satoru made, figuring that a non-sorcerer could only last 0.2 seconds within the domain without suffering aftereffects. The amount of information that everyone was exposed to was about half a year's worth, leaving everyone unconscious while still standing. This level of the domain could've allowed the cursed spirits to awaken from their trance at any moment, so Satoru focused on eliminating all one thousand of the transfigured humans targeting the civilians in just 299 seconds.