A mind cleanse was a Nebari process used to remove violent impulses from the brain, rendering the subject docile and easy to control, willing to go along with the Nebaris' plans. There were two versions of the cleansing process; a short-term system that drugged the subject with implants positioned behind their eyes to render them compliant, and a long-term process that took several cycles. Both of these methods could be broken; the long-term method could be undone if the subject is exposed to a traumatic event, such as when Captain Selto Durka reverted to his usual behaviour after Rygel attempted to blow him up, and the short-term process would eventually wear off as the drugs metabolised out of the system.
The short-term cleansing method had no effect on Nebari or Pilots, and Hynerians were comparatively less affected by it as their accelerated metabolisms caused them to burn through the drugs far faster than most other species. Crichton was also immune due to the chip Scorpius implanted in him, as the chip accelerated his body's metabolisation of the drug to burn it out of his system.