The video doesn't tell us how the modern "ECO" indicator works for Toyota's different hybrid models, because it's different for each model ( Prius, Lexus and Highlander Hybrids). What it specifies is that the principle is quite the same as the vacuum gauge and the light helps motorists to drive efficiently.
The post announces that Toyota considers this indicator a very good feature which could be installed soon in all its vehicles. This is how the further development might be like: it will include an Eco Zone Display "which would indicate the throttle opening and whether the driver is driving efficiently." For cars with automatic transmissions, it will also include an Eco Lamp that would illuminate if the driver drives efficiently enough to stay within the "Eco Zone." The equivalent of the latter for manuals will be illuminating the optimum shifting point. Add average mpg indicators and you can save 10 to 15 percent of fuel in your driving if you work to keep the lights on.
For hybrid models, the information display goes even further: the ECO indicator would also include a "Power Meter to display system output and regeneration power levels, and a Consumption Display that would include information such as Energy Collection, Average MPG, Instant MPG and Cruising Range."
[Source: Toyota Open Road Blog via Green Car Congress]