BF,
Its "fast-acting" b/c it elevates blood sugar quickly - this is the idea behind the glycemic index.
Maltodextrin is a "medium-length" complex carb, meaning that there are only several glucose (dextrose) strung together, rather than 10's or hundreds, to make the molecule. This means that the glucose can be hydrolyzed away very easily (small molecule, little cleaving to do) and the glucose is freed easily to enter the blood.
The length of the glucose chain is just 1 thing that will affect the GI. IN something like oatmeal, for instance, the carbs are within the fibrous structure of the oats, making the carb chains even less accessible for breakdown, lowering the GI.
-Randy