It's when your go out in public with a trench coat on and nothing else, open it towards a crowd of people, do a little happy dance, then close it and calmly walk away.



OR

It's when you Change the Firmware stored on a card/device,

This firmware is stored using flash, so they call it re-flash when you change the firmware.

Usually doing this allows you to use this card or item in a way that it was never intended to be used.