Here's how it works.
Your iPhone uses a capacitive touchscreen to sense when your finger taps the screen・ and that technology works by noticing when your finger distorts the electrostatic display layer on the device. That's why your iPhone is smart enough to distinguish between a pencil eraser and your finger ? your finger conducts electricity while your pencil eraser doesn't.
Using this principle・ the Halo Back team created a screen protector that includes a capacitive pathway between the top left corner of your screen (where the back button in app menus is usually located) and the bottom of your phone. This channel ends at the "invisible button" portion of the screen protector・ allowing you to tap that small area to complete the electrical current and trick your iPhone into thinking you're tapping the corner of the screen.

The best thing about the Halo Back screen protector is that it leaves the option up to you・ as you can always opt to tap the back button as you normally would ? you simply have another way to access that portion of the screen without reaching if you so desire.
Now・ it's important to note that the Halo Back is dependent on the button layout in iOS・ which can change. And while Apple has yet to make any drastic changes to where the back button is usually located in menus・ it's entirely possible.