If it is the weapon Rae Don Chong fires at the police truck in Arnold Schwarzenegger's movie Commando then that is the M202 A2 "flame rocktet" unit.
It was basically a four shot incendiary rocket launcher that was uncle sam's supposed replacement for the flamethrower. Basically it lobs these crappy incendiary rockets at it's target... the rockets don't "explode" per se as much as they burst into flame and burn for a long time. The weapon was supposed to be used to clear bunkers, trenches, buildings and whatnot without using explosives. Needless to say... it sucked. It was useless against armor and pretty much useless against hardened positions as well. The military abandoned it in the '80s.
Edit: the M202 A2 seems to have the nickname "Flash" on the internet... most likely thanks to the Soldier of Fortune game that had that weapon and called it thus. I have never heard the M202 called a "Flash" before.
WOW, thnx for the info... it's very hard to find some real pics of it.... some badass weapon