From The Stargate Omnipedia
Palm-sized unit which acts much like a human grenade. To initiate the device both ends of the detonator must be pulled, revealing an intricate mechanism on the inside, then turned and closed. A red trigger initiates a brief countdown which makes the detonator explode.
Wraith detonators can also be rigged to explode when the red indicator becomes exposed. For example, if a rock that has been depressing the trigger is removed, the device will then detonate.
The Defiant One - Sheppard, eager to use any arsenal he can get his hands on, takes detonators off a Wraith corpse.