In determining the external stability of retaining walls, failure modes like Bearing failure, Sliding and Overturning are normally considered in design.

In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. 

The principle behind shear keys is as follows:

The main purpose of installation of shear keys is to increase the  passive resistance. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance.

On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall  base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 0.8φ in the previous case and the sliding resistance can be enhanced.