First up, a little backstory for those who don't know. During the development of The Legend of Zelda: Ocarina of Time, Nintendo used Arwing models to test the flight patterns of Volvagia, alongside the Z-targeting system. This was left in the game's code, and discovered a number of years later by modders.
Since then, we've seen countless videos of people using cheats and mods to make the Arwings spawn in-game. That's what makes today's video that much more impressive. It marks the very first time that someone has gotten the Arwings to spawn in-game without using cheat codes or mods.
This player in particular got the Arwings to spawn via "arbitrary code execution." This method is used by speedrunners to force the game to load and run the save file name as if it's game code. The end result in this instance is some attacking Arwings!