Dislocation motion along a crystallographic direction is called glide or slip. In the bubble raft experiment, dislocations glide when the raft is deformed. There must be a local shear stress in an appropriate direction on the dislocation for glide to occur. Dislocation glide allows plastic deformation to occur at a much lower stress than would be required to move a whole plane of atoms past another. These animations compare how plastic shear deformation occurs in a 2D primitive square lattice with and without dislocation glide.
The stress required to cause slip by moving entire planes past one another, and the stress required to cause slip by dislocation motion can be estimated. The calculation shows that the stress required for slip is much lower when the mechanism of slip is dislocation motion, and from this we can conclude that slip does occur by dislocation motion.