Capacitive power supply
They are implementing a "charge pump" that increases the available output current. It's easier to see if you redraw the circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

When "Line" goes negative, C2 charges through D1 and D4. Q1 is cut off, as are D2 and D3.

When "Line" goes positive, it not only supplies current through D3 to the load, it also switches on Q1. This pulls the negative end of C2 up to the "Neutral" voltage, allowing it to discharge through D2 and supply additional current to the load.



Note that the same increase in available current could have been achieved with an ordinary 4-diode bridge rectifier. However, this circuit has the advantage that the negative side of the regulator remains directly tied to neutral.