On the Atkins site there are some articles about the glycemic index, check those out. Basically some foods will be converted into glucose faster then others, causing a larger insulin spike. Starches and sugars would cause an insulin spike larger then veggies or other fiber rich foods of an equal carb amount. Insulin rushes can result in cravings, weight problems and so on. The goal of Atkins is to balance blood sugars for better health.

If you stick to the induction list of acceptable foods, it should be ok. If you eat nothing but meat and then enough chocolate to equal 20g, there is a good chance you aren't going to be losing.

Some people do well with just cutting out refined sugar and flour and not watching carbs or GI. It depends on the person, how insulin resistant they are and so on.