Suppose a minimum wage of W_Min is imposed in a monopsony factor market. By law the firm can't hire workers at a wage below W_Min so the effective labor supply curve and MFC is W_Min as long as the firm hires fewer than L_Min workers (L_Min is the number of workers who would like to work at wage W_Min). If the firm were to hire more than L_Min workers it would have to pay more than the minimum wage to increase hiring. For this reason, MFC increases beyond that point.