When the capacitor has no voltage, T1 is off. R1 sends current to the base of T2. T2 conducts, dumping the current through R3 to ground and the diode has none.
When C1 has some voltage, some current flows through R1, turning on T1, but getting weaker as time goes by. If T1 ever becomes fully on, it dumps the current through R2 so T2 has no base current. T2 is off and the current through R3 flows through the diode.
As the current through R1 gets smaller and smaller, T1 turns off and R2 turns on T2 which dumps the current through R3 and the diode has none.