Automatic mode: Hold Right ⌘ and press the first letter of an app’s name to jump to it instantly. E.g. Right ⌘ + F switches to Finder.
Cycle mode: Have multiple apps open with the same initial? Press the shortcut repeatedly to cycle through them. E.g. Right ⌘ + F can cycle between Finder and FaceTime.
Manual mode: You can open the overlay with a global hotkey to switch to an app by clicking on it. Hover an app and press any character to assign it. Closed assigned apps will automatically open.