The Personality Tuck Machine was designed to alter a certain human's psyche to whatever else may be desired. It utilised "opto-genetics", altering the neurons in the brain using light.
Explaining to Rimmer what the machine did, Rimmer thought that he would like to try it and remove his cowardice. Kryten said that it would take all week. However, when Kryten explained that it would involve a laser scalpel, a drill bit and a big needle, Rimmer was out of the room in a flash.
Later, as the gang planned to hunt eight polymorphs that the Cat had given birth to, Lister volunteered to go through a procedure using the personality tuck machine to remove all his emotions. This would render him undetectable to the polymorphs, who sensed and fed on emotions, giving the gang an advantage. Rimmer thought that it had reduced Lister to a blank, vacant, vegetative state, although this was before Kryten had even turned the machine on.
The plan did not work as without his emotions, Lister began to indiscriminately kill all versions of the Cat, without first checking which was the real one. In the end, the Cat killed all his "children" himself, and Kryten presumably used the tuck machine to return Lister's emotions. ("Can of Worms", Series XI)