Active Edge, which LG originally came up with and that Google adopted with the Pixel 2. Of course they dropped it after a few devices.
It was basically a button/key press that you could configure to trigger actions by firmly applying pressure with you hands around the lower third of your phone. It gave a very satisfying haptic vibration response based on the amount of pressure you applied and you could even set the amount of pressure until it was triggered. It had something magickal about it.
If you're interested in the tech: https://www.idownloadblog.com/2017/10/19/google-pixel-2-teardown-ifixit/