When a compressive stress is applied, a force is exerted on the half plane to relieve the compressive stress conflict. This effect is similar to squeezing a watermelon seed between ones fingertips until it pops out the side. In a crystal for a dislocation to move like described above the atoms must be removed from the half plane of atoms. This requires molecular diffusion of atoms from the dislocation for the dislocation to move in the upward direction. Equivalently, vacancies must diffuse to the extra half plane.
Diffusion is a thermally activated process.
So, when a dislocation moves due to an applied normal stress the process is called thermally activated climb . Climb requires a whole line of atoms, into and out of the paper, to diffuse away from the dislocation. As mentioned above work hardening happens because defects within the solid interact with one another. One specific interaction that happens to dislocations in dislocation jogging . When a dislocation becomes jogged by the intersection with another dislocation, those dislocations have regions containing the characteristics of the other dislocation and are hence jogged.