This playthrough was made using the Dolphin emulator, which is an app that allows you to play Wii games on a computer. In order to create a TAS (tool-assisted speedrun), which is what you saw in the video, you need to use savestates. Savestates are a thing you can do using an emulator, which lets you save your game at any time. If you keep making savestates, you can replay any part of the game and fix any mistakes you've made, which is how the game was played perfectly in the video. One of the few major glitches which the Dolphin emulator has is that whenever you load a savestate, it causes Miis to have no faces. This is why the Miis in this video don't have any. Hope that made sense!
