FTG is a technique which allows the user to transport themselves to a given location in the blink of an eye.
don't ever think that minato learned by himself because,
FTG is senju jutsu and tobirama found this jutsu and he taught it to tsunade and she passed it to minato.
if someone wonder why she didn't use it against madara , i would say that she was busy healing the kages and fighting at same time and if she was alone then she will use it definitively against madara.
tsunade is senju and she also knows lighting release so definitely she knows FTG from her uncle tobirama
proof: she can do it only one time in fight and she told raikage she will save it against madara .