how does this boost circuit works?

This seem to be a boost circuit. But the switching condition is that output will be over 10V. So how does the swithing works?

That's an unusuall circuit. It looks like a flying-capacitor boost regulator.
I'm not convinced it actually works just from the circuit diagram.

When Q673 is on it allows C678 to charge up to about 4.3V from the 5V input through D670.
When Q673 is off C678 and R678 are in series with
the 5V supply.

After the drop across D671 and D672 I can't see how it could produce more than 8V at the output.
Anyone?

R678 is going to limit the available ouput current to about 1mA.

Q673 needs to turn on and off rapidly. I suspect
C678 s a feedback path to make it oscilate.

The feedback mechanism for the voltage regulation is that when the voltage on the output exceeds 10V the zenner diode starts to conduct current. 10.6V on the output will cause Q672 to conduct which turns off Q671 and in turn Q670.
When Q670 is off there is no base current to Q673 so it will be off.

skjian,

This booster circuit uses the similar principle as a voltage multiplier by means of 3 diodes.

There is a big problem here. 1N4148 is a signal diode. Using them as power diodes will "fry"them. This had to be stupid person who designed this. Perhaps he/she never had tested it before. He/she probably worked the 10V or higher, based on simulation.

One more comment. This is far too complicated circuit to use for boosting voltage. You can make a much simpler one using generic voltage multiplier.
There are many examples on the Internet.

look for the simple just simply search for power electronics and i have a principle book from the forum u just search to find the simple and basic circuit that if u want to try using boost circuit