Power Construct
Transforms 10% or 50% Zygarde into Complete Forme when its HP is below 50%.
Full effect
Transforms 10% or 50% Zygarde into Complete Forme when its HP is below 50%. This ability cannot be copied or replaced. This ability only takes effect for Zygarde.
In game description
Other Cells gather to aid when its HP becomes half or less. Then the Pokémon changes its form to Complete Forme.
Using Power Construct in Cobblemon
Abilities are passive and always active, so Power Construct costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.
See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.