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Prevents anything from working on the user next turn. Perish Song hits through Protect. Consecutive uses lower the chance of working by 50%, so the first use has a 100% success rate, and if it is used the following turn again, it has a 50% chance to work, and then it has a 25% chance to work. Detect, Endure, and Protect are all on the same counter, so if a Pokémon uses Detect, Endure, and Protect, Protect will have a 25% chance to work.
Detect is identical.