This means less fat storage (insulin is the hormone produced to cause sugars in the bloodstream to be stored as fat in your fat cells), but also in the long run it means your pancreas is less likely to over-react to the stimulus of sugary foods and therefore overproduce insulin. What this means in practice is that if you consistently follow a low GI diet then on the odd occasion when you do have a splurge it will do less damage to your waistline than for someone who is following a low GI diet.
As a very unscientific sample I can say that having followed a reasonably strict low GI diet for the month leading up to my wedding definitely limited the damage from my honeymoon. Two weeks of delicious three course meals and limitless cocktails would be sure to do alot of damage, but instead I gained a meagre pound for all my indulgences.
This isn't down to some freak genetics, I once quite impressively put on a whole 6 lbs of weight in 10 days on a trip to barcelona in my pre-low GI life! This was clear evidence of the benefit of my month of low GI eating.
This also doesn't mean I'm now free to continue eating the same way now that I'm back home. It's very easy over-time to undo the good work by regularly eating high GI foods, making your pancreas produce more and more insulin and then gradually finding it easier and easier to gain weight. The only way to keep reaping the benefits is to consistently eat low GI and keep high GI treats for special occasions.
A four week low GI strict phase is a good idea to get you started, The montignac method is a good place start.
Once you've got your blood sugar balanced you'll know it because your appetite will reduce, you'll be able to go 4-6 hours between meals without feeling grumpy or weak and you won't crave sugary foods as much, you should also have lost a couple of pounds without going hungry. At this point you can start making high GI exceptions to your diet, but try and limit these to one meal a week (a good allowance for date night or a friends birthday or other special occasion).
So if you have a trip coming up where you plan to indulge for a more than a couple of meals, lay the ground work with a couple of weeks of strict low GI eating, and then you'll be able to enjoy your indulgences without piling on the pounds.