Now, the how it works is that it some ingredients mimic insulin, some block absorption of carbohydrates, some (gymnema, for example) actually decreases cravings for sugar... Different formulas will offer different properties; some may offer all of the above, some may simply target one such action... How many carbohydrates you need to take is something I can't answer for you; it's subjective. Different users will respond differently to different carbohydrate amounts. For example, I prefer to use GDAs with a lower amount, others with a moderate amount, and some use a couple of capsules before cheat meals.
The reasons for taking a capsule prior to carbohydrates are numerous... I, for one, am sensitive to carbs. I eat pancakes and want to take a nap. I eat ice cream and I feel like sh't; lethargic, temperamental, etc. Using a GDA prevents this from happening, amazingly enough. Thing is, that carbohydrates are essential for a balanced diet and that's directly related to insulin, serum glucose, glycogen, glucagon, insulion sensitivity (to prevent metabolic syndrome), etc.
So, it's not necessarily a matter of creating a problem and minimizing the effect, but rather utilizing the carbohydrates efficiently... so, there is no problem to speak of in the first place.
Using either without carbs should only be done at certain times... I'll sometimes suggest a fasted pre-WO dose to optimize GH release, which occurs during exercise, and to also improve insulin sensitivity so that post-WO meal is soaked up rapidly. The pre-bed fasted dose, which also improves insulin sensitivity, will also optimize GH release during sleep (though don't expect body composition changes; one would need to be consistent with this dosing over a long period of time - even then it's not at all comparable to peptide use)... and another nice little perk we noticed is that it enhances sleep quality.
So, long story short... think of how important insulin is and all that it does; that's what GDAs do. It just so happens that they're very versatile in that they're side effect-free, and can be used in a number of different ways. And remember, when thinking of using an insulinomimetic without carbohydrates, that insulin has benefits and actions aside from simply disposing of blood glucose.