Insulin release is mainly a bodily response, caused by the food you eat. You are able to control this factor to a pretty large degree, just like you can control whether or not you get goose-bumps by making sure to wear a sweater when it's cold - even though the goose-bumps in and by themselves isn't something you can control. You can't choose to have high or low levels of insulin floating around in your system, but you can pretty much steer it by eating. A truckload of fast carbs (like Dextrose), a big bowl of rice, and an extended period without food will all have very different impact on your Insulin levels.
But let's not rush ahead of ourselves here - let's get the big picture together.
Fast vs. Slow carbs
As you probably know, carbs are actually plain sugar. As an experiment, chew on a piece of non-sweet bread for a while, and you will notice an increasing sweetness developing, because your saliva and jaw-action is breaking down the large chunks of sugar into smaller units, which in turn gets noticeable as they get smaller and smaller. This is how it works in your stomach as well. Now, the difference between slow and fast carbs, aka. complex and simple carbs, is how big chunks the chunks of sugar are!
Assume that you have a handful of loose powder. In your stomach, there's really not anything more to digest, so the whole bunch gets launched into the blood stream, which the whole point of eating in the first place, but in this case it all gets launched at the same time, causing a HUGE, sudden surge of blood sugar!
Complex carbs, on the other hand, is more like having a ball of yarn in your hand - it won't just "come apart" anytime soon, but needs to be stripped little by little. This equals a nice and even release of sugar into your blood stream.
Insulin to the rescue
Now, what's the big deal about how fast it goes into the blood stream you may ask? Well, as a general rule, all sudden changes involving the body is bad. This is no difference. The body takes a beating from extreme levels of blood sugar (the eyes is only one such area - diabetics often develop eye problems as they grow older, simply because their regulatory system for blood sugar is out of synch), so in order to protect itself the body releases it's first, last, and only defense against the scum of the blood stream: Mr.Insulin!
