Dynamax and Gigantamax
The giant Pokémon of Galar: how Dynamax works, Max Moves, Max Raid Battles and Gigantamax forms.
In Galar, Pokémon can swell to enormous size for three turns. This is Dynamax, and it appears in Pokémon Sword and Shield (including the two expansions) and nowhere else in the main series. Some Pokémon can go even further with a special form called Gigantamax.
Where and when
Dynamax needs a Power Spot, which is why it is limited to Galar's stadiums, the Wild Area's dens and a few other places. A Trainer with a Dynamax Band can Dynamax one Pokémon per battle, and it lasts three turns, or until the Pokémon is switched out. In Max Raid Battles, the wild boss is a Dynamax Pokémon for the entire fight, and only one of the Trainers' Pokémon can Dynamax during the raid, with the chance passing from player to player. Zacian, Zamazenta and Eternatus cannot Dynamax.
HP and Max Moves
A Dynamaxed Pokémon's HP rises by 50% at Dynamax Level 0, and a further 5% for every level up to +100% at Level 10 (you raise the level with Dynamax Candy). Its other stats, Ability and item are unchanged. When Dynamax ends, its HP drops back in proportion.
All of its moves turn into Max Moves of the same type and category. Damaging Max Moves never miss, have a side effect, and still deal a quarter of their damage through Protect. Status moves all become Max Guard, which does fully block Max Moves. The Max Move's power depends on the original move. Tackle (power 40) becomes a 90-power Max Move, Flamethrower (90) becomes 130, and Fighting and Poison Max Moves are weaker (Close Combat at 120 becomes 95).
Side effects
Max Moves carry effects. Max Flare, Max Geyser, Max Rockfall and Max Hailstorm set sun, rain, sandstorm and hail. Max Lightning, Max Overgrowth, Max Starfall and Max Mindstorm set the four terrains. Others change stats for the whole side, such as Max Strike (lowers the target's Speed), Max Knuckle (raises Attack), Max Airstream (raises Speed), Max Ooze (raises Special Attack) and Max Quake (raises Special Defense).
Protection while Dynamaxed
A Dynamaxed Pokémon cannot flinch, cannot be forced out by moves like Roar, and is unaffected by Encore, Torment and Disable. It ignores weight-based moves (Low Kick, Heavy Slam), one-hit knockout moves and Destiny Bond. The Choice items stop working while it is Dynamaxed, and Assault Vest still stops it using Max Guard.
Gigantamax
Some individual Pokémon have a Gigantamax Factor. When they Dynamax, they change shape into a Gigantamax form and use an exclusive G-Max Move for one of their types. Gigantamax is otherwise the same as Dynamax. There are 32 species that can Gigantamax and 33 Gigantamax forms (Urshifu has two).
The Factor cannot be bred. Wild Pokémon only get it in Max Raid Battles, but the Isle of Armor expansion added Max Soup, which can add it to nearly any Gigantamax-capable species (and remove it if it already has it). Pikachu, Meowth and Eevee with the Factor cannot evolve, and along with Duraludon they cannot be sent to other core series games, so they stay in Sword and Shield.
Why only Galar?
The lore says Dynamax comes from energy that leaked from Eternatus after it fell to the Pokémon world in an asteroid about 20,000 years ago, and caused a black storm called the Darkest Day roughly 3,000 years before the games. The phenomenon is still known only in Galar, though a Melmetal legend says it once happened in a distant land.