Obviously you'll have to look into it for yourself for a deeper understanding, but I believe some of the "magic" to it is when and how you fuel. Fats can take longer to metabolise than a big, fat hit of sugar, so that demands feeding earlier. I imagine fuelling up a margin of time before running, then maybe snacks earlier in the run/event than you would take carbs, in order for them to get going in your system.
Some margin of protein also metabolises into glucose.
I just googled "low carb marathon training" and had loads of results, including this one:
http://www.runnersworld.com/newswire/do-low-carb-diets-improve-endurance-performance
Happy reading.