Z-Moves are a brand new element to Pokemon battling in Sun and Moon and can be considered a trainer's ultimate trump card. They 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 Pokemon Types, from the Normal-type Normalium Z to the Fairy-type Fairium Z. Not only that, but there are exclusive Z-Crystals for certain Pokemon species, such as Pikanium Z for Pikachu.
Preparations for a Z-Move
Before you can use a Z-Move, you must first make a Pokemon hold a Z-Crystal whose type matches at least one of the Pokemon's learned moves. For example, if you have an Electrium Z, you can give it to Pikachu if it has an Electric-type move, such as Thunderbolt.
Conveniently, a single Z-Crystal can be held by multiple Pokemon, which can be useful if type variety isn't a concern for you.
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 Pokemon'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 Pokemon'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 Pokemon 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.
Important: You can only use one Z-Move during an entire battle--and this applies for your entire party. Hey, we did say it was a trump card!
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 Pokemon who's difficult to take down normally--during the story, usually the final Pokemon 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 Pokemon resists 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.