The link is a small metallic device which acts as a direct neural interface and was worn at the temple by the inhabitants of P3X-289. The link was controlled by the same computer system which maintained the dome in which the population lived to protect themselves from the toxic environment of the planet. Since the computer contained the sum total of the society's knowledge and wisdom, with thousands of historical books on file covering history, science, and culture, any individual could instantly and directly download information whenever it was needed. However, the link was also able to control thought, and to alter knowledge, perceptions, and memories.
When the power supply for the dome had begun to fail, the computer had compensated by gradually shrinking the size of the dome and using the link to instruct random individuals to walk beyond the dome to their deaths in order to maintain population control. Massive programming revisions through the link then altered the memories of the remaining inhabitants so that the individuals were never missed.
The population had considered the link to be a benevolent technological advantage, and Evalla had removed hers and offered it to SG-1, assuring them that it was safe. However, when a computer revision convinced the inhabitants that disconnecting from the link would mean instant death, SG-1 had to use ancient written records to convince Pallan that his memories were being altered. He agreed to manually reprogram the system so that the population became aware of the threat to their community, and the inhabitants chose to remove the link and relocate to a new homeworld.
Cross Reference: Councilman, Councilwoman, Evalla, Kendrick, Nevin, P3X-289, Pallan
Episode Reference: Revisions