A Pokémon with the Shadow Tag Ability prevents its opponent from running away or switching out. The opponent remains trapped for that turn if the Pokémon with Shadow Tag switches out. The Hold Item Shed Shell, or the moves Baton Pass, Volt Switch or U-turn will still enable substitution.
Shadow Tag doesn't have an effect in the Overworld.
Pokémon With Shadow Tag
Generation 5 Hidden Ability
NOTE: Shadow Tag was replaced by Infiltrator as the Hidden Ability for these three Pokémon in Generation 6.