Sturdy prevents the Pokémon from being knocked out from one hit. It instead leaves the Pokemon at 1 HP if it would have been knocked out in one hit from full HP and the Display will say "<name> held on with its sturdiness"

Pokémon with this Ability

Pokemon Type
411MS Bastiodon RockSteel
525MS Boldore Rock
232MS Donphan Ground
074MS Geodude GroundRock
526MS Gigalith Rock
076MS Golem GroundRock
075MS Graveler GroundRock
081MS Magnemite ElectricSteel
082MS Magneton ElectricSteel
462MS Magnezone ElectricSteel
299MS Nosepass Rock
476MS Probopass RockSteel
524MS Roggenrola Rock
410MS Shieldon Rock
213MS Shuckle BugRock
227MS Skarmory SteelFlying
208MS Steelix GroundSteel