How does the screen-off-gesture-detection work?

I'm wondering how the feature in the OPO that recognize screen gestures, while the screen of the device is off, actually works? For example, the feature to double tap to awake the phone, or draw a circle to open the camera app. Does the screen really been turned off, or actually the screen is on, and just the screen is black. (this way the screen is on, hence can recognize the gestures, and on the other hand save battery energy).
Thanks!