Someone mentioned this to me not long ago about Futaba transmitters, and which applies to the Futaba 6XAS radio (and some others too, possibly all of them).

They have a 'Battery failsafe' mode, so that during your flight, if the receiver detects that the receiver battery is below a certain voltage for over a few seconds, it will automatically cut the throttle to a high idle. This warns the pilot that it's very much time to land the plane. From what I've been told, this probably only applies to Futaba radio systems.

It's not mentioned in the transmitter's manual, but it can be found on the website.