Here's something I wish I had never learned: in Swordand Shield, your shiny Pokémon can have one of two particle effects when they pop out of a Poké Ball. Some Pokémon have dazzling little stars, while others have squares. It's a bit of blink-and-you'll-miss-it visual flair. Nothing more, nothing less.
And yet, knowing how worked up people get over shiny hunting – to say nothing of Poké stats, in general – I feel like the floodgates just opened. Some poor soul is going to try collecting them all.
Watch this video for a comparison featuring a "star shiny" Chewtle and a "square shiny" Sirfetch'd. It's the kind of thing you could go through life never noticing unless someone specifically pointed it out.
How does the new shiny variation work? That's still being fully worked out.
In essence, it's more randomness, but it's believed that if you're chaining (catching the same Pokémon again and again to improve your odds), then your shiny Pokémon will most likely come with the square sparkle animation. If you're breeding Pokémon, your shiny will likely have the normal star effect.
"Shiny Pokémon rarity has been split," Twitter user Kaphotics explained after peering into the code. "Same 1:4096 odds to obtain a shiny, but there are two types of shiny. 15/16 show stars, 1/16 show squares. Square rarity is 1 in 65536!" However, "in a surprising twist, it looks like chained shinies that are destined to be shiny are forced to be shiny in such a way that [results in squares]."
"There could be more factors at play, and different situations for breeding. Time will tell."
Do what you will with this information. I'll settle for ashiny Pokémon, to be honest.