There are two ways to start a fast in KAI:
Option 1: Start now
Go to your Daily Log (today's date)
Scroll to the Fasting card
Tap Start 16h fast now
The fast begins immediately using your default duration.
Option 2: Start from your last meal
Go to your Daily Log
Tap Start from last meal below the fasting button
Adjust the time if needed (you can go back to yesterday)
Tap Start fast
This is useful if you forgot to start your fast when you finished eating.