The first time I fasted was an accident when I was mountaineering. I woke up late for an alpine start and my crew was already ready to go. I threw on a headlamp and ran after them in the dark without breakfast or a snack for later. We climbed two technical fourteeners and a thirteener that day, and I'd never felt better. Sure I was a little hungry, but I was flying up and down those mountains. I couldn't believe how much more energy I had. Now that's just how I roll.
I fast during the day, while out riding or not. I've found mental and physical performance gains this way, but it might not be for everybody. I used to bring a chunk of meat in my pocket, but now I just wait. MCT oil does help my stomach feel better without anything in it. Eating fat keeps you in a fat burning state and doesn't break the fast.
I can't tell you how often my friends will suck after stopping for lunch. Maybe it's the beers and bloodys, but I think the food plays a large role in the lethargy they experience. I know that they probably only have a handful of runs in them after they eat too. Meanwhile, I'm raging around the mountain racking up hot laps.