When you switch off the power supply, C5 and C3 will remain charged for a certain period of time. Since C5 is much larger than C3, it is very likely that C3 will be empty while C5 is still charged. If C3 is empty, U2 ceases to function, T5 will be open and the full voltage across C5 will appear on the output terminals! The protective circuitry around T4 will prevent this. While U2 receives its supply voltage T4 remains closed and everything works as it should. However, when U2 loses its supply voltage, T4 opens. In that case, T5 closes due to a current flow in R18 and D5. This will make sure that the output voltage remains 0V.