The Devo Chamber involves strapping people onto an automatic chair, binding them by the arms to prevent escape. Afterward, when the knob to the time period of the character's ancestors is set, the victim is reeled away before the chair is launched up into a de-evolution device, exposing the head to rays designed to forcibly de-evolve the person's head down to the size indicated. Koopa commonly uses this as a form of punishment.
Koopa used this to create Goombas to serve him as a dumb, but loyal police force, as well as to reduce extreme offenders to primordial ooze. When he threatened to do this to Mario and Luigi, however, they turned the tables on him and subjected him to the process himself; though it ended prematurely, Koopa began to develop more reptilian traits from that period onward.
Koopa later subjected Iggy and Spike, his loyal minions and cousins, to the Devo Chamber after becoming fed up with their stupidity and incompetence; however, he used the process to evolve them to the "Advanced" level, vastly increasing their intelligence. However, this backfired on him when Iggy and Spike, now able to think for themselves, instead turned on him and joined forces with the Mario Brothers to put an end to his rule.
"Devo 4" was a separate Devo Chamber where Koopa kept King Bowser, who he has de-evolved into a giant sentient fungus.