A powerful, wearable technology created by a long-dead race called the Atonieks. The armbands enhance a wearer's natural abilities, including strength and speed. According to the Tok'ra Anise, who spent much of her career studying them, the devices predate Goa'uld history.
The armbands lock around a subject's forearm, though with no apparent way to unlock and remove them. They then inject the wearer with a specially-designed virus (possibly nanite-based) which allows the armband to interface with and enhance his or her physiology.
The technology is imperfect, however, and will automatically deactivate and fall off after the body's immune system has managed to defeat the interface virus. This discovery -- when SG-1 was in the field of combat serving as test subjects -- led Anise to the conclusion that the weapon meant to enhance the Atoniek armies may have finally been responsible for their downfall.
Upgrades - The Tok'ra Anise arrives at Stargate Command to use SG-1 as test subjects for the Atoniek armbands.