Once you have created and saved your event, you can go back in to it and access the Advanced features to set up Auto Release on specific ticket types.
To do this, you will need to go to Edit your event and select Ticket Types
Click on the gear icon beside the ticket type that will be going on sale first
Ex. You would like Tier 2 to be released when Tier 1 hits a specific threshold so you will set the auto release on the Tier 1 ticket type
Select Auto Release from the menu at the top of the pop-up
You will then input the release limit on the current ticket type, and which ticket type will be released once it has been hit and select Add
Once you have selected Add, you will see the auto-release displayed above where you initially added it. If you want it removed, simply select the X beside it
Once your auto release is set, the next tier of tickets will be auto released once the threshold you set has been met