Zippy Zap inflicts damage and will always result in a critical hit. It has a priority of +2, so it is used before all moves that do not have increased priority.
Generation VIII onwards
Zippy Zap cannot be selected in a battle. However, it has been modified: its PP was changed to 10 while its power was changed to 80. Additionally, its secondary effect is raising evasion by one stage, instead of a guaranteed critical hit.