The Thief gets the Thief's Reflexes feature which states:
When you reach 17th level, you have become adept at laying ambushes and quickly escaping danger. You can take two turns during the first round of any combat. You take your first turn at your normal initiative and your second turn at your initiative minus 10. You can't use this feature when you are surprised.
I'm wondering whether you can opt not to use this, such as when damage at the start/end of your turn would reduce you to 0 HP, or if you've put a condition on an enemy that lasts until the end of your next turn. For example, the pyrotechnics spell blinds creatures until the start of your next turn.
I'm unsure because the wording both says "You can take two turns [...]" and "You take your first turn [...]", where the first perhaps shows that it is optional but the second seems to say that it is not.