Z-Moves are a new element to Pokémon battling introduced in the previous games: Sun and Moon. Essentially, they can be considered a trainer’s ultimate trump card. Z-Moves can be utilized once you’ve obtained the Z-Ring during the main story and one of many Z-Crystals.
There is a Z-Crystal for each of the 18 Pokémon Types, from the Normal-type Normalium Z to the Fairy-type Fairium Z. Not only that, but there are exclusive Z-Crystals for certain Pokémon species, such as Pikanium Z for Pikachu.
Once in battle, you can use a Z-Move by selecting (or tapping) the big “Z-Power” button towards the left side of the bottom (touch) screen while selecting a Pokémon’s moves. This will change all of the Pokemon’s moves that match the Z-Crystal’s type into an extremely powerful Z-Move.
In general, the stronger the Pokémon’s original move, the stronger the resulting Z-Move. So a Z-Move triggered from Thunderbolt will be stronger than one triggered from the lesser Thundershock. Even better, Z-Moves can deal some damage to Pokémon guarding themselves with moves like Protect or Detect.
Meanwhile, status moves that don’t inflict damage can still trigger Z-Moves, but they work slightly differently. Depending on the status move itself, the Z-Move version of the status move will grant a special effect, such as a stat boost or health recovery, before triggering the usual effect.
Because you can only use a single Z-Move per battle, it’s obviously very important when you choose to use it. Perhaps the best time to unleash an offensive Z-Move is when facing a particular tough Pokémon who’s difficult to take down normally–during the story, this is usually the final Pokémon in a trainer’s party.
Of course, remember to keep in mind Type Match-ups when selecting your Z-Move as there’s nothing worse than seeing an absolute spectacle of a move failing to make a noticeable dent because the opposing Pokémon resists it (or worse, is immune to it).
For status Z-Moves, the timing depends on the special effect granted. If it’s health recovery, you obviously want to use it when at low health. Or if it’s a stat boost, you’ll probably want to use it early on to shift the momentum in your favour.