The Willhunters are nanites that are used to invade a victim's nervous system. The Willhunters suppress a victim's conscience and reprogram the person's mind. It leaves the person in a catatonic state and unable to act without orders from the Manhunters or the Cyborg-Superman, Hank Henshaw. The Willhunters somehow have the ability to read a victim's mistakes and regrets and trigger these memories. Flooding the victim's mind with these images may be how the Willhunters are able to break down that person's will and alter their thoughts.
Willhunters were used on Green Lantern Hal Jordan when he and Guy Gardner invaded the Manhunter homeworld of Biot to rescue the Lost Lanterns. The nanites flooded Hal's mind with such memories as Hal becoming a pilot against his mother's wishes, moving to Evergreen City, Washington in an attempt to run away from a commited relationship with Carol Ferris, not wearing his power ring when flying his jet and watching Jillian Pearlman being taken prisoner after that jet crashed, and having to attend former lover and fellow Lantern Arisia's funeral.
It is not known whether it was the emotions wrought by these memories or the willpower stored within the power ring, but Jordan was able to push through the Willhunters' programming, effectively burning them out and freeing his senses to continue the fight with Hank Henshaw.