In a nutshell: Remember when so many games relied on button mashing? It's not used quite as much these days, but some gamers still consider themselves experts---though it's unlikely any are as good as Ludwig Ahgren.
On his Twitch channel, named Ludwig, the streamer used Mario Party 4 to show off his incredibly fast button mashing skills (it includes some NSFW language). Released for the GameCube back in 2002, the interactive board game consists of minigames, one of which, Domination, involves repeatedly hitting a button as many times as possible in a time limit.
A lot of people have preferred techniques when it comes to button mashing, such as rubbing or using two fingers. Ahgren's is pretty weird, allowing the controller to bounce around quite a bit---but it obviously works well.
In Domination, each button press creates a new Thwomp. Once the time is up, they all fall like Dominoes as the number of Thwomps is counted. Ahgren had already reached a previous high score of 199, but this run saw him hit 201. His line is so long that it extends beyond the end of the platform and into the sky.
Three years ago, someone uploaded a TAS (Tool-Assisted Speedrun) of the same minigame. It got a score of 160, which is the maximum limit for the non-modified version of the game. The modified version Ahgren was playing allows scores above 160.
Ahgren now says he is the "fastest masher in the world." It's hard to disagree.