How does this simple circuit work?
If I did not oversee anything, it should work as follows:
Assume that the peak voltage of the source is 10V.
phase 1 (Upper terminal of the source is 10V)
Capacitor is charged to:
$$V_C = 10V - V_D $$
phase 2 (Lower terminal of the source is 10V)
Gate of the Mosfet &quot;sees&quot; the supply voltage + the charged voltage across capacitor, hence
$$V_G=V_i + V_C$$
At the same time, the lower diode is foward biased, hence current flows through the MOSFET, now active.
UPDATE
Here is some Illustration of the voltage levels:

NOTE: The above assumptions do not consider the time required for charging and the sinusoidal shaped source, therefore the peak voltage levels might differ.