The hack is called "fast and easy way to totally destroy a battery". Charging the capacitor in the flash is basically dunping a short circuit on the battery for a very short time (until the capacitor has received some charge). Doing that to an almost discharged Lithium Ion battery could drop the battery below the cutoff point - turning it into a package of hazardous waste that will never work as a battery again. (Lithium is hazardous waste, which is why you're told to never throw a cellphone battery in the trash.) How low can the battery get before that happens? If depends on temperature, humidity, the condition of the flash capacitor, the amount of charge left in the charge capacitor - many things. 25% is a safe point at which to stop allowing the flash capacitor to be charged.
If you don't care about dropping $15 (for a new battery) every time you use the flash at the wrong time, keep looking for the hack. I'd rather buy a spare battery and keep it fully charged and in a plastic bag in my pocket.
