After an even earlier adopter tried it, I had to also way back when...
Turned off the key on my 2001 Prius. It basically went to IG-ON and Neutral (although the gear shift lever was still in D). Unfortunately, to turn the Prius back on, you have to be in Park or a physical neutral, so you have to slow down, pull over, and turn the key again.
However, I'll note that if the Classic Prius is already on, and you turn the key to Start again, the computers basically reboot (splash screen comes back on the MFD). A "normal" car would have nasty grinding/screaching starter motor sounds...