Pokémon Gold Version, Silver Version, and Crystal Version - Switching Out
After that, Pokémon that chose to switch out do so.
When a Pokémon enters the battle, the effect of Spikes applies for that Pokémon and that Pokémon faints if Spikes causes the Pokémon to have zero HP. If both sides have Spikes and both Pokémon are switched out, here is what happens:
Leader’s Pokémon switches out --> Leader’s Pokémon loses HP because of Spikes --> Other player’s Pokémon switches out --> Other player’s Pokémon loses HP because of Spikes --> Pokémon with zero HP faint