The device was a technological machine created with the intention of accelerating the evolutionary process of a Human being. This worked on a genetic level and resulted in a mutation of the subject's genome, which provided numerous enhanced abilities such as healing, telekinesis, telepathy, super hearing and so on. The intelligence of the subject dramatically increased as well and evolved the body to the point where it was capable of Ascension. Brain scans showed a level of synaptic interaction much higher than normal. (SGA: "Tao of Rodney")
Unfortunately, the device had a fatal flaw: the changes wrought did not include the mental requirements necessary for ascension. As such, if an individual subjected to the machine did not attain the mental concentration needed to let go of their physical form, then the genetic modifications would eventually kill them in the process. As a result, the machine contained a great deal of information on human physiology and could reverse the changes made so long as a previous template existed for reference. (SGA: "Tao of Rodney")
The machine itself appeared as a platform with a computer terminal that had access to a view screen, where a user could analyze the data. Once activated, a device built on the ceiling directly above the terminal engulfed the user in a spiral of green energy, after which the subject would unconscious as the changes to their body began to take effect. (SGA: "Tao of Rodney")
- It's likely that the DNA resequencer was either the predecessor or successor of this device.
The Lanteans were responsible for the creation of the ascension machine, who not only used it in their experiments to attain Ascension, but also as a potential escape from the Wraith. (SGA: "Hide and Seek")
Thousands of years later the device was discovered by the Atlantis expedition, and was unwittingly activated by Dr. Meredith Rodney McKay, the Chief Science Officer of the Expedition. The device did what it was supposed to and put McKay on the path of Ascension, giving him new superpowers; however it was soon discovered that the changes to his body required that he either learn to ascend or die. Ultimately, Dr. Carson Beckett was given the necessary information by McKay on how to reverse the modifications, and he was returned to his previous physiological state. (SGA: "Tao of Rodney")