In the Martial Arts Arena, Kazemori Masaki and Minato Namikaze were training in the Flying Thunder God Technique. Masaki was still using a Shadow Clone. Aside from his original body's administrative duties, having a "Kazemori Masaki" with limited power conduct the training was always the better option.
After all, no matter how powerful he might become in the future, Minato was still young and nowhere near his prime.
Even the Fourth Hokage from another timeline probably could not defeat Masaki, much less Minato, who was only seventeen.
Still, after becoming Masaki's student, Minato's strength had indeed grown rapidly. He had also entered his growth spurt, shooting up like bamboo nourished by abundant nutrients. By now, he had become a young man.
Standing before Masaki, Minato tightly gripped a Flying Thunder God Special Kunai. Shaped like a trident, it was heavier than an ordinary kunai and more powerful. Once infused with wind release chakra, its destructive power was extraordinary.
Minato watched Masaki, reached into his Ninja Tool Pouch, and hurled a barrage of Flying Thunder God Kunai. Yet as they approached Masaki, they seemed to strike an invisible wall, veering past him and landing elsewhere.
Minato had known all along that this could not hurt Masaki. He simply needed the Flying Thunder God Kunai to land so he could expand his range of movement.
Of course, if Masaki wanted to, he could destroy every kunai with his Wind Release. He had the strength to do it, even with a Shadow Clone possessing only half his power. But that would have been far too unfair to a youngster like Minato.
The gaps in their age, experience, and strength were plain to see. If Masaki fought seriously the entire time, Minato would have no chance of winning, defeating the purpose of the training.