To answer IK's question first: I don't know the exact gram to gram conversion for dex carbs but it's about 1.25 grams of carbs per teaspoon of dex.
You're right most people follow up the PWO shake with basically a real meal about 45 minutes to an hour later. As far as the high insulin spike the idea is that immediately after a weight workout (provided it's a good one, lol) the muscles are very insulin sensitive and bascially primed to take up carbs and nutrients. So the idea there is that being in that insulin sensitive state is going to help you avoid that "crash". See, since you are in a primed insulin sensitive state less insulin does the job. Your body doesn't have to release huge amounts in a desperate attempt to bring blood sugar levels down to normal resulting in a temporary dip below normal and thus the "sugar crash". Of course a crash can still happen if you do it wrong. It can be just too much carbs or it still can be absorbed TOO fast.
I'm no expert on all of that but hopefully that is the basic jist plus the added advantage that the body is in a muslce fueling state rather than a fat fueling.
That's where the amount of carbs comes in, though. There is still going to be an amount of carbs that can overwhelm you and result in a crash so that if you find yourself getting sleepy or some other symptoms around a half hour later that would be a clue to adjust the carbs down. If you sip some of your carbs during the workout that would change the amount also.
The meal later is not to combat a crash but simply to continue to fuel the ongoing recovery process and the obvious reasons why a bodyuilder woud eat
Like you said though, it's just theory and it may not be for everyone. I will say that I am fairly sensitive to dex and malto but I never have problems with my pre or post and they are simply made up of whey isolate, dextrose, malto and creatine.
If I were to take in the same thing but not around a workout I would crash big time.
In the end you gotta find what works for you. Most people I know use the high GI carbs around their workout very successfully but I'll be the first to say that there is not one way for everyone! Again, as I've said before, a combination of dex and malto is better and will end up getting glucose to the blood FASTER than just dex alone plus alleviating some of the bloating that many will get from having all their carbs as dextrose.
Personally for my preworkout shake I use the standard shake I have been doing for years with protein, malto, dex (and some creatine) but I only have half before my workout and I sip the rest during. Not because I need to have that for just an hour or so of workout but because doing it that way keeps me from crashing. I don't take in too much at once and then I take it in slowly and steadily.
I was just making sure about the 80 grams carbs versus 80 grams dextrose thing.
I have noticed that many people are very confused about the digestive process. Especially the gastric emptying part. I think people actually think that if the take in dex along with oatmeal for instance, that the dex will somehow be magically separated out and absorbed first since it is "high GI" and then the oatmeal will come later. Of course the stomach doesn't have compartmetalized emptying and basically the more solids you have in their the slower everything empties in a UNIFORM way. Plus fat, etc.
I actually read where some person was advocating milk as the ideal pre and post workout drink since the whey would absorb very quickly to fuel the muscles while the casein would bring up the rear and also that it was quick carbs (untrue). Really the most ludicrous thing I've ever heard. That's just someone not being able to face the fact that whey protein powder does have an advantage in this.