This is a topic we should have in Frequently ASked Questions - the section just after this one. If you look in that section Questions Section there is a dissertation about this near the end.
Basically Atkins severely limits carbs during induction and adds them back very slowly. Most people go into ketosis during the two week period and sometimes have headaches. The CAD and CALP programs limit carbs for two meals but allows them for the third meal. Since you still do have carbs, you do not go into ketosis and headaches are infrequent. Both plans stress drinking 8 or more glasses of water per day.
The best suggestion for a complete understanding is to borrow the books and read them several times. I'd be willing to send you my CALP, if you like. Just PM me and I'll pop it into Canada Post!
I'm sure the rest of the Chicks will add their two cents to this thread.
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