"Eco" light in 2013 Yaris
 

What does it actually signify?

It doesn't appear to be related to load or torque converter lockup, only a throttle pedal position.

It also disappears when using the gearbox to slow down (which, by watching the fuel consumption display, is the only time I get over-run cut happening).

Any thoughts?

My impression from driving a Corolla is that it's largely RPM related, encouraging you to keep the revs down.

So it's completely bogus then