Many people bring up the argument that FTG isn't really speed because it teleports. Yet, Minato is known as the fastest ninja alive, so it has to be related to speed. So I'm theorizing how it works and why it's known as speed.
FTG doesn't have any flashes or ripples like kamui. It's also been described as flying. I think there's a reason for this.
FTG is a space-time jutsu. Speed is distance traveled divided by time. In other words space-time controls speed. I think that FTG uses space-time to launch someone at infinite speeds. Or, It might might put the user in something like hyperspace from star wars. A side effect is that it allows someone to travel through objects.
What do you think about my idea on how FTG works. Be respectful and don't forget to rate.