"Low Glycemic Index" (associated in broad terms with fats, proteins, some complex carbohydrates) simply means "difficult to digest" – measured relative to glucose, one of the easiest foods to digest with an index of 100. This is a good thing when sitting on your couch, because your body doesn't need an immediate blood sugar spike, unless you're trying to understand the plot of "Inception". Here, it's an advantage (and much healthier) to digest the food slowly, and release energy slowly.
Under continuous exercise, while your body is actively burning energy, it's quite appropriate, and perfectly healthy, to consume small continuous amounts of high-GI foods – since "high GI" means precisely "easy and quick to digest". There's no benefit here to "slow release" – that just means your body is expending unnecessary energy and effort on digesting the food. Under continuous exercise, the ideal solution might be to bypass your digestive system altogether and hang a glucose drip directly into your bloodstream!
(Disclaimer – I haven't found or read any of the original research for maltodextrin below; most of this is gleaned from promotional blurbs, no idea how strong the evidence is for any of the explanations.)
Maltodextrin is unusual. As mentioned, it's technically a complex carbohydrate – it is polymer chains of glucose (glucose and dextrose are synonymous). However, it apparently has a HIGHER Glycemic Index than glucose. This is strange, since the digestion process would first require the body to break down the maltodextrin polymer into glucose monomers, requiring time and energy input. The explanation seems to be that maltodextrin polymers are absorbed faster than an equivalent number of individual glucose monomers, and that after absorption the polymer bonds are easily broken.
The reason for the rapid absorption of maltodextrin relates to osmolarity — which means the concentration of solute molecules/ions in a water solution. In order to deliver 10 glucose molecules to the digestive system, you could eat 10 glucose monomers or a single maltodextrin polymer with chain length of 10. Since osmolarity is determined by the number of intact molecules/ions in solution, regardless of size, the maltodextrin solution would have lower osmolarity – one tenth the osmolarity of the glucose solution. It's not intuitively clear to a non-chemist why the number of molecules matters, regardless of size, but that's the way it is.
Solutions of high osmolarity tend to draw in water. In your stomach, a high-osmolarity glucose solution with draw in more water than a low-osmolarity maltodextrin solution. This is proposed as the explanation for the empirical observation that maltodextrin is absorbed faster and more easily than glucose, and that large amounts of glucose (or other simple sugars) can cause stomach upset in athletes.
It's also subjectively important that maltodextrin does not taste sweet, so you can eat a lot of high-GI carbohydrate without a constant sickly-sweet taste. It's super-sticky though – as I learned from trying to repackage Power Bars; or just see what happens to maltodextrin powder if it gets even slightly damp.
