Basically the Pokemon with this ability gets right up to the opponent's face and the opponent needs more energy, out of panic, to use an attack, because of how aggressive the Pokemon with Pressure are, making their opponent more exhausted. Which makes a lot of sense when you look at Aerodactyl, Vespiquen, Weavile, and most Legendaries XD they use their opponent's stamina to their advantage and make their opponent feel like they are under fire constantly and they need more energy to survive.