From Prius chat
...The Owner's Manual sort of explains it on pg. 247. The Prius (including the Plug-in) is drive by wire, which means that the accelerator pedal does not mechanically control anything. Instead, a sensor measures the amount the pedal is depressed and software decides how to map the the accelerator pedal depression to how engine/motor speed. The modes simply change this mapping. ECO mode makes it the least sensitive (more pedal depression required for the same acceleration), normal (neither ECO nor Power mode) makes it moderatively sensitive, and Power mode makes it the most sensitive (less pedal depression for the same acceleration). In all the modes, if you put the pedal to the metal, the amount of acceleration is the same.
And follow up:
Not quite. Since the end points are the same, all that they do is MOVE the sensitive section of the pedal response. In ECO mode, the sensitivity is at the early portion, in Power mode, at the later portion. Normal is a more linear response curve.
With the exception of AC controls, a talented foot could have the car perform EXACTLY the same in any of the modes. If you don't have one of those talented feet, choose the mode the gets you the highest MPG (many find that the ECO mode does this, others find Power mode works better for them).
Read more: http://priuschat.com/threads/eco-vs-power-mode.110875/#ixzz32fDD0z6H
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Interesting about the Atkinson Cycle. The actual compression ratio is 8:1, and the expansion ratio is 13:1. That's why it can run on lower octane. 13:1 compression would probably require 100 octane.