With this piece I intend to clarify Desmond's mind time travel; addressing the subject of consciousness, the function of The Constant and what happens if no Constant is in place. My intention is to keep this as brief as I can, as clear as I can, and as easy-to-read as I can.
First let's consider consciousness. Apparently, when a mind travels to a different time (think of Desmond turning the Fail Safe in the Swan Station and 'waking up' covered in paint in Penny's flat) one consciousness takes over the other consciousness. Except "takes over" isn't quite right; I think it's more like a fusion where both consciousness' mesh together, with one consciousness taking precedence over the other. This merging of consciousness' is fundamental to understanding the whole principle so I'll do my best to spell this out.
Desmond in Penny's flat provides a perfect example. One moment 2004 Desmond is turning a Fail Safe, the next moment he is zapped back in time to Penny's flat (approximately 1996) where 1996 Desmond absorbs this 2004 consciousness. So the 1996 consciousness is well aware of who he is, where he is and when he is, and yet 1996 Desmond is partially aware of his 2004 consciousness. The microwave sounds like The Swan Station alarm. The model yacht in Widmore's office seems familiar. Slowly the 2004 consciousness takes firmer awareness (remembers Charlie, remembers Jimmy Lennon and the cricket bat) and Des' brain, fused with two consciousness', is able to reconcile the disparity.
One smack to the head later and 1996 Desmond and 2004 Desmond have worked out their differences and snapped back to the place they were supposed to be. No Constant necessary this time! Because both consciousness' managed to retain a hold of themselves, anchored by memories, the "side effect" issues don't appear.
So that was Flashes Before Your Eyes. Let's apply this 'fused consciousness' principle to events in The Constant. As we saw, 2004 Desmond was in a helicopter and then suddenly snapped back to 1996, in the army. 1996 Desmond woke up (that is, the 1996 consciousness) and assimilated, or fused, the memories and experiences of 2004 Desmond as if it were a vivid dream.
Desmond - "I'm sorry, sir. I was...I was having a dream, sir. . . I was in a helicopter, sir. And there was a storm, sir. And I don't remember the rest, sir."
This is where 2004 Desmond's consciousness was relegated to; the status of a faded dream. The two minds had merged, but the 1996 Desmond's consciousness took precedence entirely. In Flashes Before Your Eyes the two consciousness' had managed to mesh yet retain a level of awareness. In The Constant the 2004 Desmond was consumed completely by his 1996 counterpart. This is why, when Desmond returned to the helicopter, he had no idea how he got there or who Sayid was. This is why Desmond needed a Constant.
OK. I think we're doing well. Once the fused consciousness idea is grasped the rest of this a piece of cake. See, as we saw in Flashes Before Your Eyes, so long as you are aware of the dual, fused consciousness - that is, you know a part of your mind belongs to a different time - then you will get reset (a cricket bat should do the trick!) and put back to how you were. However, if you're like Desmond in The Constant, who had no awareness of his 2004 counterpart, you need to 'awaken' that dormant mind to produce dual consciousness.
That's all it is. In The Constant 1996 Desmond kickstarted his dormant 2004 consciousness by using his love for Penny to spark it into life. We even see the moment it happens. Whilst on The Freighter, when Des is talking to Penny in that memorably moving declaration of unyielding love, the scene is cut to show 1996 Desmond walking away from Penny's flat. He looks back, to her window, and smiles to himself. That smile, that was the awareness returning, I think. That was 2004 Desmond waking up, the memories coming back. The shared consciousness was understood and both 1996 and 2004 minds were reconciled and balance was restored.
There. Not so difficult to grasp. Those that like their endings happy can abandon ship here. For everyone else, let's finsh up by considering what it's like when you don't have a Constant and your consciousness decides to go time travelling. In short, let's consider what happened to Minkowski.
When 2004 Desmond went back to 1996 Desmond, it was 1996 Desmond that took priority of his consciousness. Minkowski, however, went the other way; his 2004 consciousness retained priority over the consciousness of whatever historical counterpart he was transported to. And so he found himself in a similar condition to Desmond in Flashes Before Your Eyes, experiencing his life with knowledge of the future. Desmond experienced that for a couple of days and it sent him fairly crazed; Minkowski had to deal with it for an awful lot longer. . .
Minkowski - "I was just on a ferris wheel."
The way I see it Minkowski would have been retreading events he previously lived through, and then occasionally snapping back to The Freighter to catch up with what had been happening whilst he had zoned out. Interestingly the issue of changing his fate may have occurred to him. That is, he may have decided to try and not wind up being a crew member onboard The Freighter whilst he was retreading his past. If he succeeded, that would create a paradox and would explain his death (the universe doesn't collapse in a paradox, as with Eloise the rat it's simply the 'thing' that created the paradox that gets wiped out).
Minkowski - "It's getting harder. It starts happening faster, too. . . I can't get back!"
However, as this piece of dialogue suggests, Course Correction kicked in and, no matter what he did Minkowski would end up on The Freighter, the same way Desmond would always end up back on the Island. And so, yo-yoing between the past and the present, with the past catching up with the present, eventually he may have been reliving boarding The Freighter, and pinging back to being on The Freighter with Sayid and Desmond. . . Faster and faster. . . Eventually his brain wouldn't know the difference, the yo-yo effect too acute for his brain to comprehend. And so: brain vessels explode and blood comes out of your nose and you die.
And that, I think, is pretty much that. And there I was thinking this was going to be tricky to explain. . .