Shock absorbers are stitched in a fashion so that, if they are fallen on, the stitching is pulled out and the force placed on the piece of protection is reduced. As such they are commonly favored in situations where you're relying on marginal protection or when using ice screws that can blow under low forces. As they can only be 'activated' once, they are best used where the chances of a fall are slim but the consequences of gear failure could be serious.
Shock absorbers are specialist extenders for marginal placements of the sort usually encountered while aid climbing or winter climbing. They generally consist of slings sewn up with stitches that are designed to rip under load so that the impact of the load is gradual rather than sudden. They provide added protection in climbing and rescue situations where they not only absorb energy directly because of the stitch ripping effect, but they also allow your rope to absorb more energy from the fall by increasing the time interval of the fall. The standard shock absorber (for instance) can effectively reduce peak loads by 3-4kN in any climbing or rescue system.
Sports and active leisure stores "Gandrs" offer work and sports shock absorbers from such manufacturers as Singing Rock, Petzl and Edelrid. Stay safe during your vertical activities and reach the highest peaks together with "Gandrs"!