Just learned about this item. The sites I’ve checked only have a vague description of it. I’m guessing it’s not a held item due to Showdown not containing it. Seems like you throw it at an opposing Pokémon to lock them in a move. How much is damage increased with the Dumpling and how is the recoil calculated?
You use the Choice Dumpling on your own Pokémon in battle after it has used a move to give them the 'Fixated' status, which means they do more damage when continuing to use that move in a row, but also take more damage.
Bulbapedia page for Choice Dumpling
This Smogon page gives the damage dealt buff as 1.5x, and the damage taken as 1.33x
