The Volsinii were large devices designed to sustain as well as preserve those who were encased in them. Tentacles would plug into the users temple, abdomen and spine where they would provide the necessary life support functions while the user's mind entered a virtual reality environment.
While inside the pod, the user was in a state of suspended animation and was unaware of the world around them. The mind itself would exist in the virtual world run by the Keeper. People were capable of dying within the virtual environment of the pods. (SG1: "The Gamekeeper", "Avatar") (RPG: "Friends and Foes: Stargate Season Two")
Dr. Bill Lee and other scientists developed a computer training game program for personnel which they tested on themselves. However, when the game was played by Teal'c with his extensive field experience, it was found lacking. The nature of the chairs' programming learned from the user, and Lee requested Teal'c continue to play the scenario to improve it. Unfortunately, the game quickly became too tough for Teal'c, drawing on his memories of combat to increase the danger he faced in each scenario, to the point that the game would not even let Teal'c leave via the game failsafe as it recognized that Teal'c would normally never quit. Teal'c's continued failures in the game resulted in him receiving sudden jolts each time he lost, causing spikes in his adrenaline production, which, in turn, caused his blood pressure and heart rate to rise dangerously, to the point that Teal'c would eventually suffer a heart attack if he remained in the system. Lee and Lt. Colonel Samantha Carter, inspired by a comment by Brigadier General Jack O'Neill, exploited a fluke in the system by connecting a second pod up to Teal'c's system while removing the data recorder from the chair. This gave Dr. Daniel Jackson- chosen to prevent the game drawing on a soldier's tactical expertise and becoming even more difficult to defeat- a two second advantage, allowing him to perceive events in the game two seconds in advance. This gave him the chance to join the game and give Teal'c vital information. After several attempts, they were able to defeat the scenario and the pod deactivated. This technology is later successfully adapted for virtual reality training of SG team members. (SG1: "Avatar") (SGA: "Doppleganger")
The crew of the Lantean ship Aurora were all put in stasis pods and then infiltrated by a Wraith, which somehow managed to kill one of the crew and take her place in the pod, its image in the virtual reality being replaced by the image of the dead crewmember. The Wraith convinced the crew that they had never put themselves in stasis in the first place, and also tried to discover the secrets of Lantean technology contained on board. When Lt. Colonel John Sheppard entered the virtual reality he was able to convince the Aurora crew of the truth, the Wraith being killed before the Aurora destroyed itself to destroy two Wraith cruisers. (SGA: "Aurora")
When Doctor Rodney McKay is infected with the entity from the Crystal of M3X-387, Colonel Samantha Carter comes up with a plan to use the Volsinii virtual reality technology to connect McKay and Lt. Colonel John Sheppard's minds. According to Carter, it only took a minor adjustment to change the technology from connecting two minds to a virtual reality to connecting one mind to the other's subconscious. In the years since Teal'c got trapped in the virtual reality, the SGC has been able to modify the technology so that its much more compact and the users only need to have a pair of leads attached to their heads rather than sitting in a pod. (SGA: "Doppleganger")
When Ronon Dex and Lt. Colonel John Sheppard traveled to Earth for the funeral of Patrick Sheppard, they were assisted by Ava Dixon in tracking down and defeating Poole's Replicator that had been created illegally by Dr. Richard Poole. In an unexpected development, it was revealed that Ava was in fact a Replicator herself. Since she had helped them, but because of the security risk associated by letting a Replicator remain free, her consciousness was downloaded into a Virtual reality pod. (SGA: "Outcast")
When the energy version of Dr. Elizabeth Weir came back to Atlantis after the Asurans failed attempt at digital ascension, the expedition considered putting her consciousness into a VR pod as well. Ultimately, it was decided for the remaining Asurans, as well as Weir herself, to be sent through a Spacegate. (SGA: "Ghost in the Machine")