You can also choose a Shortcut to launch. And Shortcuts can do anything from launching an app to running JavaScript. So you can imagine the possibility...
Personally, I have a double-tap set to reveal Control Center and a triple tap set to initiate open a new note in my note-taking app, Drafts.
To open a specific app, you will first need to make a Shortcut that performs the “Open App” action and then select that Shortcut from the available options in the Back Tap settings. To do that, open the Shortcuts app (pre-installed on every iPhone or available from the App Store on older versions of iOS).
Once in Shortcuts, create a new one with the plus icon in the upper right. Name your shortcut if you want (by pressing the three-dots “More” button), and then press “Add Action.” There is an overwhelming number of options if you are unfamiliar with Shortcuts, so just use the search and look for the action called “Open App.” Select this action from the search results and then a block will appear with a blue “Choose” option where you can choose the app you want it to open. Choose your tuner of choice.
Once saved, this Shortcut will be available as an option in the Back Tap settings.
***Note: The Tonal Energy app actually allows you to set up Shortcuts that jump to specific places within the app like the Analysis or Metronome section. You can find this in the TE settings. It will save you a bunch of extra taps.