Re: Flashcubes?!
Xenon flash with the sensor shut down after the appropriate integrated dose of light has been sent. This is done by pumping the output of the sensor (which is proportional to the reflected light from the subject+background) into a capacitor, and the cap voltage is used to shut off the flash output using a FET across the tube (or something like that, don't have the circuit in front of me).
Those cheap flashes are quite interesting to work on (AFTER the flash cap has been safely discharged, of course).