The Dynamax Band is a white wristband with some red and blue highlights. It can sometimes be a technological counterpart to Z-Ring, due to have some similar design parts, as well as having similar systems as Z-Ring and Key Stones.
It allows the player's Pokémon to Dynamax or Gigantamax. Each Trainer can only Dynamax a Pokémon once per battle.
Ash Ketchum acquire his own Dynamax Band from Leon so he his Giganatamax his Pikachu in their battle. After their battle, Leon let Ash keep the band as see him as a good unrefined trainer. Unlike his Z-Ring and Z-Power Ring he wore in his journey in Alola, Ash only wears his Dynamax Band when he is in Galar while also participate in battles in the region.