A 1n4148 diode makes the transistor on and off?

[walid1]

I've built this emergency lighting circuit
When the main voltage 230 V is present, LEDs turn off
And when does not LEDs turn on
Transistor and 1n4148 diode are responsible for making the LEDs turn off
Question: How do the 1n4148 diode makes the transistor on and off?
Thanks

note: the burnt R = 5.6k
When I implemented the circuit it worked very well