It simply charges up two capacitors in parallel, usually from a third much larger capacitor, then ‘flies’ the second capacitor into a series arrangement with the first capacitor, hence giving twice the peak voltage. This then charges a fourth capacitor up to this voltage and keeps it there by switching at about 10kHz.

Compared with traditional switch-mode PSUs, there is less stress on the transistors, but the capacitors do age. In theory you are limited to multiples of the input voltage, but you can do tricks using smaller flying capacitors to achieve other voltages.