I am not really familiar with the harware of your phone, but as far as I know, double tap wake requires a piezoelectric sensor, which xiaomi phones don't have. In your case, I would think that the kernel simply kept the touch screen active while phone is asleep. It works, but it consumes more battery.Double tap to wake or swipe to wake also would do the trick. I have got this feature on my Motorola Xoom with a custom kernel , without the required hardware.