Shields work on their own. You don't need to turn anything on: they're used automatically when a scheduled day goes by without a check-off.
When it happens
The check runs when you open the app, when you come back to it, at midnight, and in the background for widgets. It looks at the scheduled days you missed, up to yesterday.
The whole-gap rule
- If you have enough shields to cover every missed day, they're all used and the streak survives. You'll see Shield used, and those days show as Shielded in the habit's calendar.
- If you're even one shield short, none are spent and the streak ends. You keep your shields for later.
Example: you missed Tuesday and Wednesday and have 1 shield. One shield can't cover two days, so the streak resets and your shield is saved.
More than one habit? If there aren't enough shields for all of them, the habit with the longest streak is protected first.
A few more details:
- Rest days are never missed, so they never use a shield.
- A shielded day keeps the streak alive but doesn't add a day to it.
- Archived habits never use shields.
See also: How to earn streak shields