From a 'strength of materials' viewpoint, a spoke may be approximated by a cylinder of material required to stand a given stress. The spoke is pretensioned, and this load is reduced as the spoke comes to the bottom of the wheel under load as illustrated below.

The pre-tension in the spoke is about half the yield stress of its material and is produced by rotating the threaded nipple and pulling spoke material into the wheel rim. To ensure that the wheel 'runs true' the tension in all the spokes must be the same.

From: McMahon & Graham, "The Bicycle & the Walkman," Merion (1992)

The spokes remain in their elastic range both as pre-tentioned and during road use. As the diagram shows, road shocks decrease the spoke load below the value on a smooth road. Increased tension due to such a shock is shared a by the spokes opposing the shock force.