If you're new to Digit (welcome!), your shiny new Digit account will activate and start saving for you automatically once Digit finishes analyzing your checking account. There is no action needed from you to "turn on" auto-savings!
Keep in mind, Digit looks for optimal times to save for you based on your unique financial situation, so it may take a few days before you see your first auto-savings transfer, but hopefully you'll be up and saving shortly.
Digit will save for you automatically any time it sees a safe time to do so, based on your checking balance, upcoming expenses, and patterns in spending habits.