When we look at images of edge or screw dislocation, it seems as if the direction in which that dislocation will move is already fixed by the 'way the dislocation is present'. For example, take any image of edge dislocation, and say the top half is moving with respect to the bottom.
Dislocations will move in the direction of slip direction and on the slip plane. That is the dislocation motion direction is dictated by the slip system of that crystal (FCC, BCC or HCP)
Hence, only in the case when the plane between the top and bottom is also a slip plane, things work out. But what happens when the plane between the top and the bottom set of atoms is not a slip plane for that particular crystal?