When the switch opens the falling edge will cause a negative pulse on the 4013 input. The diode removes that negative pulse.
Input to 4013 without diode –one pulse (on/off) –zero volts is the centre line. The signal goes negative to minus 1.44 volts. Positive pulse comes from direct connection to +3 volts,negative pulse is smaller because capacitor has to discharge through 1M resistor.
Input to 4013 with the diode –the negative signal is almost totally removed. The signal goes negative to minus 0.35 volts.
