Android, it might make sense to post it here.
When holding a phone, your index finger is almost always resting at the back towards the top of the phone. Then why not use it to control the upper part of the screen FROM THE BACK?
Phone manufacturers can make (part of) the phone’s back touch-sensitive. For example imagine using the glass section at the back of Pixel phones as a touchpad to pull down the notification shade and toggle quick settings, without the need for finger acrobatics to reach the top of the phone.
It can even support gestures such as double tapping. To avoid accidental touches it can be made pressure sensitive.
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Some phones with rear-mounted fingerprint scanners implemented a feature like this when you could swipe on the sensor to do various things. But having a proper touchpad or touch-sensitive part on the back of the phone would be far more reliable and useful.