Question about naked short selling:

[brentspeak]

I understand that "clothed" (or conventional) short selling involves an external party (the short seller) borrowing shares from an unknowing investor (via a brokerage) for the purpose of selling those borrowed shares to another third party (where the short seller will eventually have to buy shares back again to make the original investor from whose shares were borrowed whole again, etc.)

However, "naked" short sellers sell shares that haven't been borrowed. If the shares in question haven't been borrowed, how and from whom are naked short sellers obtaining their shares?