And of course, the
At 14% we hit the hard close stop, which actually keeps the butterfly
open just the slightest crack to prevent binding against the throat.
teardown page for how this is done.
The 17% rest position is the balance between the two concentric return springs, which allows the engine to produce a very small amount of power for "limp" mode if the throttle mechanism fails.
A good diagnostic for throttle functionality is to listen for the PWM drive to the throttle motor, and feel the ECM actively fight you when you try to turn the flap. When the engine is shut down the ECM holds the flap near the rest position, but not necessarily exactly at it. The motor is driven in either direction until the desired feedback value is reached, without much regard for the springs.
In real life, the Prius seems to never get past 50% throttle opening and under most driving conditions, remains down between 20 and 40 percent. This isn't exactly snapping the throttle full open, but the operating regime does keep the engine under fairly high torque load [thus efficient running] most of the time. This chart shows a collection of recorded data in a mixed city/highway driving segment. The little "bump" around 2500 RPM is interesting, and is still under investigation for its efficiency implications.
So far, fooling around with applying slight biases to throttle position feedback to try and force it farther open or closed has been tried, but not with any obviously productive results.