Effects
Secret Power has a 30% chance of inducing a secondary effect on the opponent, depending on the environment. The move's animation also changes, reusing that of a specific attack for each terrain.
Location ,Effect ,Animation
Grass,Sleep, Needle Arm
Sand or regular terrain, Accuracy reduced one stage, Mud Slap
Caves or rocky terrain, Flinching, Rock Throw
Water, Attack lowered one stage, Water Pulse
Snow or ice ,Freezing, Avalanche
Buildings Paralysis ,Body Slam
Outside of battle
In Generation 3, Secret Power allowed the user to create their own secret base when used on weeping willow-like trees.
Source
