A Pokémon with the Pressure Ability forces the opponent to use 2 PP per attack instead of 1, disregarding whether that move hits or not.
If a Pokémon with the Pressure Ability is in the lead spot of the party, the odds of finding a higher-level Wild Pokémon falls by 50%.
Pokémon With Pressure
Related Threadspile on pressure point (downtown) - last post by @ Mar 14, 2003
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Pressure Plate? - last post @ Dec 30, 2002