Great little circuit, but how does it work? - PWM filter.bmp (0/1)

[Jack J]

Here is a LPF that smooth out PWM pulses. The PWM frequency is about
200Hz. It's response time is fast while doing an excellent job
filtering out the 200Hz PWM pulses.

I know the circuit works but I am not sure how. Can anyone recognize
or figure out how this works?

Thanks,

J