If I Tideturner an Overzealous Fan on a Showdown, can I use his effect?

Yes, you can use Overzealous Fan's effect after swapping it with Tideturner during a showdown.

According to RiftJudge FAQ 9047, swapping Overzealous Fan to a different battlefield does not cause its ability to "whiff" because the unit remains on the battlefield, which is the only requirement for paying the "kill me" cost.

Sequence of Events

1. Play Tideturner: You play Tideturner from Hidden as a reaction. This starts a chain.
2. Tideturner Resolves: Tideturner enters the battlefield, and its "When you play me" ability triggers. You swap Tideturner with Overzealous Fan.
3. Fan's Ability: If Overzealous Fan has the "Defender" designation (e.g., it was moved into the combat), its "When I defend" ability triggers.
4. Resolution: When the chain reaches the Fan's ability, you may choose to kill the Fan to move the attacking unit to its base. Because the Fan is still on the battlefield after the swap, it remains a valid target for its own cost.

Important Notes:

• Targeting: The target of the Fan's ability (the attacking unit) is locked when the ability is placed on the chain. Moving the Fan does not invalidate this target.
• Defender Designation: Per RiftJudge FAQ 9174, if you swap a unit into the defender position during a showdown, it gains the "Defender" designation during the cleanup phase following the resolution of the swap, which then triggers the "When I defend" ability.
• No Whiff: As long as the Fan remains on the battlefield (even if it is at a different location), the ability can be resolved successfully.

This is my interpretation based on the mechanics of triggered abilities and battlefield presence. Official FAQs 9047 and 9174 support this interaction.