How does the S flip case work? I know that the magnets turn the phone on and off, but what about the time/date display on the case? Does the case somehow use the phone's battery to display those? If so, does it not drain the battery faster?
This data is attributed to an online manual for the device at this location:

http://gadgetguideonline.com/s8/galaxy-s8-led-wallet-cover-guide/

NFC powers the device, and is estimated to have minimal draw to the phone battery.

See the quote from the forum thread below the manual:




  No, you cannot turn off the display. The LED cover is powered through
  NFC. Once you snap the cover on, it will drain power. But power
  required is very marginal. It should not affect the battery life of
  the phone.