In a larger array we have put 5 vacancies in a column in the center of the crystal. When you run you see the relaxation of the atoms around the vacancies. Apply a shear force of 0.4. Change the sign of the shear force to run the movie backwards. (If this runs painfully slowly, use the configure dialogue box to reduce the size of the array.)
The row of vacancies is equivalent to a short length of edge dislocation with its ends fixed in the blue layer. It is analogous (though not quite the same) as a Frank-Read source. It does generate dislocation loops in essentially the same way.