The Kor Mak bracelets are Goa'uld technology, designed to create a physiological bond between the wearers. Teal'c had heard of the devices in passing long ago and believed that it was Cronus who used the technology when a prisoner of value needed to be transported by one of his Jaffa. Once the pair of bracelets had been affixed to the Jaffa and his prisoner, if the two became separated for more than a short period of time, both would become ill and die. If only one died or was killed, the other would die as well, just as if they had been separated, thus the prisoner could not escape, and the Jaffa would be punished for his incompetence.
Vala had stolen a pair of the Kor Mak bracelets from Arlos, and she brought them to the SGC where she described them as having been the ceremonial marriage bracelets of the Egyptian sky goddess Nut. When she placed one bracelet on her own wrist and one on Daniel's, the two became linked, and if they were separated beyond a range of approximately a hundred feet for more than half an hour, both became ill and collapsed. Vala hadn't known that the illness would affect both of them, nevertheless she refused to remove the bracelets until SG-1 had claimed an Ancient treasure and she had acquired her share of the loot. The devices gave off a barely measurable energy signature, and Dr. Lee attempted to remove them with an electric saw, but he destroyed three blades without success.
With no alternative, SG-1 brought Vala with them when they went to Glastonbury to search for the Ancient treasure. Daniel and Vala were still linked when they volunteered to test the Ancient communication terminal which transferred their consciousnesses to the bodies of Harrid and Sallis in the Ori galaxy. When Vala died in the Ori galaxy, her body in the SGC also died, and Daniel might have died as well had the Prior not restored Vala's life. After the terminal was destroyed and Daniel and Vala recovered, Vala used a small handheld device and passed it over each of the bracelets, releasing them.
Vala left through the stargate, but about an hour after she had gone, Daniel collapsed and Vala was returned. The bracelets were not meant to sustain the physiological bond after they were removed, but Dr. Lee theorized that the Ancient communication terminal could have been a factor and the combination of the two technologies could have created a more permanent bond. SG-1 turned to Arlos, from whom Vala had stolen the bracelets, hoping that he might be able to sever the connection. However, Vala had also stolen a necklace from him, and he refused to help unless the necklace was returned. Vala had traded the necklace to Inago for a power coil, which she traded to Caius for his cargo ship, which had come into the possession of the Lucian Alliance. SG-1 had to recover the cargo ship from the Lucian Alliance, exchange the ship for the power coil, and trade the power coil for the necklace, which they then returned to Arlos. However, Arlos had no solution to offer, and suggested only that the effect of the bracelets should wear off eventually on its own.
In fact, the effect of the bracelets did begin to wear off gradually, but the team was unwilling to risk separation through gate travel, and so Vala accompanied SG-1 on their missions to P8X-412 and Kallana. When Vala was drawn through the singularity into the Ori galaxy, Daniel once again collapsed, but although he was weakened, the bond of the bracelets had faded, and he was able to regain his strength and make a complete recovery.
Cross Reference: Ancient Communication Terminal, Arlos, Caius, Inago, Daniel Jackson, Vala Mal Doran
Episode Reference: Avalon, Origin, The Ties That Bind, The Powers That Be, Beachhead