How does FTG work
The flying thunder god technique works by using chakra as an anchor. Each of those Kunai have Minato's chakra in them and they are anchors for him. Now because he is stronger than the tiny amounts of chakra that rests in the kunais he can stay fine, but if he lets himself go and let a kunai far away pull him, the pull when it takes him is instantaneous, he is transported at the speed of light.
Well you can't really leave chakra out there by itself otherwise it would just disappear. You cant really store chakra in stuff unless you're in direct contact with them once they're released the chakra will start disappearing so the seal is to store your chakra for when you need to use it.
Due to Minato's exceptional chakra control he was able to distinguish his chakra even when stored in something else outside his body, but even that was limited by distance.
The seal on the kunai is a complex yet simple inverted seal. Inverted so that his chakra inside could not be sensed by anybody, complex so that no one could break it and so that he could sense it no matter how far away it was and simple so that he could use it fast and easily without any problems.
Not really my theory, i found it in a prediction, modified it a little and decided to share it. Hope you enjoy it.