By attaching an activated mimetic device on the body of another person, the device created a holographic "hull" around the wearer, which allowed him/her to appear as the person, whose appearance was stored in the device. It also allowed the wearer to speak in the same voice as the mimicked person.
The device also had a powerful self-destruct ability built into itself.
As the effect of the device was holographic, it was unable to alter the fingerprints of the user. It also failed to disguise injuries, causing wounded Stragoth to bleed purple blood even while disguised. Most importantly, it was highly susceptible to a particular sonic frequency that disrupted the illusion as long as it lasted. (SG1: "Foothold", "Smoke and Mirrors")
The mimetic device was first used by the race known as Stragoth to gain a foothold in Stargate Command. Thanks to this device they were able to copy nearly every person on the base and they also planned to take over The Pentagon. Their plan was however thwarted by Major Samantha Carter. (SG1: "Foothold")
About three years later several of the devices were stolen by rogue NID agents who used them to frame Colonel Jack O'Neill for the assassination of then Senator Robert Kinsey. (SG1: "Smoke and Mirrors")