*Warning - comments on Jack's death scene*But it's not really a "flashback." It's what Ennis thinks happened. Although it's ambiguous, I do tend to think that is what happened.
Since I am definitely of the opinion that Jack was in fact killed, much like we see in that scene in the film, I too have tended to think of it as a sort of "flashback"; - that is: A combination of the real event and the horrific scene as played out by Ennis's imagination.
But we know that Jack's fate will remain ambiguous to us and to Ennis.... and so the scene that we see in the film is Ennis's grief and fear given life, breath and motion, and not a faithful representation of what actually happened to Jack. The events and elements in the scene do clearly underline that. I've steeled myself to watch it in slow motion:
There's the hat falling off as Jack is knocked viciously to the ground. The black hat is a Jack symbol not only to us, but to Ennis; - the hat being removed through violence indicates that a part of what makes Jack *Jack* is being removed. Hatless, he's symbolically being rendered vulnerable, unprotected, helpless.
After watching the vicious beating from a distance Ennis’s inner eye moves to close-up. One assailant proceeds to stomp viciously down on Jack's crotch, echoing Ennis's memory of the torment Earl was subjected to. And then comes the tire iron, striking Jack across the face, - Jack's beloved face beaten, rendered bloody, almost unrecognizable. Jack’s face seen “upside-down” adds further to the disoriented, nauseous feeling of the scene. All of it happens on a patch of vibrantly green grass, eerily reminiscent of the lush surroundings where J&E’s used to meet for their “fishing trips”.
The hat, the face, the genitals, the vibrant greens of nature: Concrete physical elements, the recollection of which would clearly speak to Ennis of Jack, speak of their relationship and their love and passion. And here in this violent, blurry scene it’s all under lethal attack from those faceless malevolent assailants Ennis has been waiting for, all of it being visually and intimately tied to Ennis's recollection of what happened to Earl. Though the indications are subtle (even if the scene in itself is not), and the whole scene flies by in a blur, leaving a horrified emotional impression but few concrete or detailed visuals, it seems certain from the elements in the scene that we are witnessing Ennis's deepest fears brought to life by his imagination….. doing its very worst. And Jack’s hat flying off is one of the first indicators, perhaps.