Just to clarify, there are two separate technologies being mixed up
here.

The one that is already being used (eg on Chiltern) is for a bar code
to be sent to your mobile phone. This bar code will be read by a bar
code reader on the gateline or by a gripper with a hand-held bar code
reader. At the moment, for various reasons, this is really only
workable with pre-booked tickets so that, for example, if your battery
goes dead you are on a printed manifest that the on-train staff will
have.

The second technology will be to use a chip inside your mobile phone
which will take the place of (and remove the need for) a separate
piece of plastic called a smartcard. You will wave your phone over a
smartcard reader on the gateline in the same way that you wave a
smartcard. It doesn't matter if your battery goes flat during the
journey - the power to read the chip comes from the reader (just as
you don't have a battery in your Oyster card).