A small dislocation loop with b = a/2[1 1 1] is moving on a (110) plane in an bcc crystal. The loop is being expanded by the shear stress. At some stage, the shear stress tends to move the dislocation on the intersecting (1 0 1) plane.
The part of the dislocation loop intersecting the (1 0 1) plane is a pure screw dislocation. Since the line and Burgus vector of a screw dislocation are parallel to each other, the screw dislocation may glide on any of the family of crystallographically equivalent planes that contain these vectors. Hence, it can slip on the (1 0 1) plane, which belongs to the (110) family.
It is possible for double cross slip to occur, which allows the screw dislocation to bypass any obstacles on their original slip plane.