My guess as to why warm-ups help is that the circulatory system pretty much sends blood to the brain at a more-or-less constant rate, and will do almost anything to maintain this rate. If you cause a few brief, hypoxic episodes, I suspect that a trained body will react by diverting some blood flow from peripheral tissue to core tissue. After a good warm-up, I can definitely feel a profound change with the most pronounced symptom being that may arms, legs etc. suddenly get very cold about 3:00 into a hold.
I think that a secondary effect is that the central receptors get desensitized to the lower pH. If you have slightly acidic blood then the relative change from a breath-hold is reduced (i.e. ratio of the number of ions at the end of the hold divided by the number at start).
The downside of a warmup is that they almost certainly result in O2 deficits in tissue and that these deficits deepen with each warm-up hold.
I'm going to guess that folks that have natural talent and/or train a lot get the shift little or no wram-up and that they are already so tolerant to CO2 that they don't need any tricks to dull the central receptors.
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