Wishing pieces are a useful item in Pokémon Sword and Shield. You can use them to toss into gray dens to lure a Dynamax Pokémon out of hiding and engage them in a battle. Unfortunately, you won’t get any Watts for this confrontation, but it provides an extra fight beyond the standard Pokémon dens you can spawn in the game.
They’re handy if you’re looking to farm Dittos with high IVs because they increase their chances of discovering the Pokémon later in the game.
You have two methods of finding them. The first, most straightforward of the two, purchase them from Watts trainers. These trainers are found throughout the Wild Area, in the black-and-white uniforms. You can speak to them, and they should have a Wishing Piece for sale for 3,000 Watts. There are a few methods to farm Watts, and it takes some time to do so, but 3,000 is not too much of a price tag for a useful item.
The other way you can locate this item is by the shiny sparkles on the ground. Some of them are scattered throughout the Wild Area, and if you approach them and add them to your bag, you could find one for free. You can’t count on it, though, so you’re better off traveling around the Wild Area and farming for Watts.
When you have a few of them, toss them into a dormant den and lure a Dynamax Pokémon to your location. After you toss the item in, the game asks if you’d like to save your progress, and you do need to say yes. If you don’t, the item is not used, and the Dynamax Pokémon doesn’t show up for the Max Raid battle.