Having done some more reading I think I now understand the principles. You get a boarding pass on your smartphone and it then uses near field communication, or bluetooth, to send those details to the bag tag. The bag tag uses "e-ink" (similar to Kindle?) and displays the required bar code.
So the bag sortation system continues to use bar code reading to route the bag to the correct aircraft. This means that while dropping the bag could be quicker than the present methods it won't change or improve the movement of the bag through the system.
Having realised that the tag still uses bar codes to sort the bags raises some issues. The paper tag used today ends up with the bar code on both sides of the tag once it's attached to the handle of the suitcase. Is the new tag double sided? I've also had additional, small, bar codes stuck on the top of the case which I've always assumed helps the reader systems should the main tag fail to read and there's no suggestion these extra tags will still be used.
I'd be interested to see some more technical data. I've found this
Designworks which is from the Designworks people BA have partnered with but it's just a press release.