That is not an adequate replacement for chop chop.
The screen needs to be on for the pixel gesture to work. Chop chop can be done with the screen off
Tapping the back of the phone to the beat of music will turn on the flashlight by accident all the time. Chop chop is a very deliberate gesture
Tapping does not always register. Sometimes I need to do it 2 or 3 times to get the flashlight to turn on/off, the moto gestures were almost 100% reliable for me