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Boosts the user's Defense by one stage. Doubles the power of Ice Ball and Rollout for as long as the user is in the battle. This effect is not stackable. The Defense boost can be passed by Baton Pass, but the Ice Ball / Rollout boost cannot.
There is not much competitive value in this move. It's outclassed by other moves that raise the Defense stat faster, like Acid Armor or Iron Defense. In short, there is literally no situation in which you could ever use this move.
Acid Armor is a Poison-type equivalent that raises Defense 2 stages.
Barrier is a Psychic-type equivalent that raises Defense 2 stages.
Iron Defense is a Steel-type equivalent that raises Defense 2 stages.
Cotton Guard is a Grass-type equivalent that raises Defense 3 stages.