When gases ?flashback? the flame regresses from outside (off of) the tip in use into the torch body itself. That flame will head to the point where the gases are mixed. The flame will continue to burn at that mixing point as long as fuel and oxygen are present and allowed to flow. Virtually all torches in flashback mode will ?whistle, howl, screech,? etc. If the operator does NOTHING, the torch will begin to destroy itself in a matter of seconds, with the flame burning through the torch at some weak point. In addition, if unchecked, the flame may continue to migrate upstream seeking fuel/oxygen to continue burning. Ultimately, that ?flame front? could end up at the gas source itself destroying parts and pieces along the way. Even if the fuel gas is shut off, with oxygen still flowing, the ?guts? of the torch can continue to burn.