The figure on the left shows a group of cells  in the undifferentiated state - they all have the potential to differentiate in the same way, and they all signal to each other to repress differentiation. 

The figure in the center shows that as individual cells (in blue) begin to produce more inhibitor signal through random fluctuation begin to differentiate and suppress the differentiation of surrounding cells.  This can be achieved not only  by increasing the production of the inhibitory signal but also by decreasing the synthesis of its receptor. The spacing of the differentiated cells would be regulated by the range of the signal and the strength of its effect.