It likes to make its shell thicker by adding layersof tree bark. The additional weight doesn't bother it.
When this Pokémon is at full HP, any hit that would knock it out will instead leave it with 1 HP. Regardless of its current HP, it is also immune to the one-hit KO moves: fissure, guillotine, horn drill, and sheer cold. If this Pokémon is holding a focus sash, this ability takes precedence and the item will not be consumed.