In Battle Use
When an animal with the Overgrow ability has its health drop to 1/3 of its original hit points, the Overgrow ability is activated. This ability powers up all Grass type moves the animal has by 150%.
This ability is likely to have been based off the way animals portray brute strength or force within their battles in order to become victorious against their opponents. The Grass type animals may use their additional stored energy to power up their Grass type attacks when they are in a pinch.