The devil woman's conniving tactic to (re)insert themselves into a man's life via sexual practice on a bro.
Dubbed flashbang because it is comparable to the disorienting nature of a flashbang grenade. One does not see or think clearly for a temporary amount of time after said flashbanging has resulted upon them.
This is often done by a "Ratchet" Ex-girlfriend who doesn't deserve to be with the said bro and shortly afterwords begins to weasel their way back into a comfortable status quo so they can suck all life from the target and all bros (and bras) of said target.
to flashbang :
"Dude, did you hear what happened with Randy and Galinda?"
"We've talked about this bro. Those two names are never to be mentioned in the same sentence."