Cross Slip in an fcc crystal
A screw dislocation with b = a/2[1 1 0] is moving on a (111) plane in an fcc crystal. By Frank's rule, it is energetically favorable for the screw dislocation to dissociate into two partial dislocations in the fcc crystal, forming a local constriction.
The screw dislocation, along with the local constriction, continues to glide in the (111) slip plane.
In order to cross slip to the (111) plane, the partial dislocations have to recombine into a perfect screw dislocation.