Justsu explanation.

"Very well, look into my eyes…" She said quietly as she walked closer and closer. Naruto leaned a bit forwards to get a closer look…
"…Izanami."
When Naruto blinked, he found himself in a surreal setting.
"Uhh…we're not in Konoha anymore, are we?"
Mugyuu…
He was standing in the middle of a vast canyon, which reached as far as his eyes showed him. It was completely desolate, save for the hundreds ?no, thousands? of towering mesas that made him feel like he was in the gods' very own Pachinko parlor. The gigantic spires of rough, natural sandstone looked down upon him like mute colossi, awing and intimidating him.

///

"This is the phantasmagoria, boya. A world hidden within the gaps in reality and shaped by my will. This is a world where no other god can reach; the world where I am God."
When she was standing some fifteen meters away from Naruto four black tails emerged from her lower back, each slightly over a meter long.
"For the next twenty-four hours, we fight. You will feel no hunger or thirst, so we will fight non-stop.

///

"What this means is that, every day for the next seven days, we will spent three days within my Izanami. The alternate realm exists outside of the normal time flow, so that's not an issue. You will have some troubles at the end of every three-day cycle, but it's nothing a strong guy like you can't handle."