For example, the placement of the power button on the LG Nexus 4 makes it very convenient to press when I’m done talking – either with my thumb, if I’m holding my phone up with my right hand; or with my index finger, if I’m holding my phone with my left hand. Of course, physical hardware buttons are also useful if your hands are wet or greasy, and you don’t necessarily want to touch the screen.
To enable this feature, simply navigate to Settings > Accessibility > Power button ends call. Make sure that’s checked, and you should be good to go.