It isn’t fiber. They say because of its molecular weight it is broken down later in the digestive system and somehow avoids an insulin response.
There is a lot of information on the interwebz that can explain better than I can. Peter Attia has a whole series of blog posts and there are videos by Jeff Volek (he has become involved with the company somehow).
I have tried running fasted and with some food about an hour pre-run. If I wake up not hungry, I can do up to about three miles fasted as long as I have my salt pre-run. If I wake up hungry, I will have the salt and a little bacon or some keto toast to have something in my stomach.
The longest I’ve run is 2 hours and 30 minutes. I have to have the salt and snack beforehand, but I don’t get hungry during the run, and I’m frequently not hungry afterwards.
As usual, it’s a “you need to find what works for you” situation.
Edith
I did a half marathon 2 weeks ago in training as a long run on a fasting day. Felt light, fast and fantastic! Almost beat my HM race PB just during a long “slow” run. 
I swim or walk most days 15-odd hours fasted and if I do an extended fast I just carry on. My fastest swim times have all been set on day 2 of a fast.
I prefer fasted, with black coffee that drink on my way to the gym. I feel more mobile and less lethargic if my stomach is empty. I ran a half-marathon fasted and was completely fine. It takes getting used to. Some people like to have a fat bomb and coffee as a pre-workout snack. I do not think that there is a right way to do it, just depends on your preferences and your goals.
prefer fasted every time … perfect to start fresh and can do hours on that and thats how it should be