The ENABLE signal is any other logic that you want as an interlock. Q turns true IF A) both buttons are pushed within the discrepancy time AND B) ENABLE is true.
Under what conditions do you want the operator to be able to actuate things? If Estop has been pressed? If the light curtain is tripped? You depending on how you program, you could potentially put conditions like that somewhere else either instead of or in addition to here.