I have been wondering but was afraid to try it.

 

Nothing should happen, if there is a time you need to kill the engine while driving, you need to hold down the on/off button

someone confirm?

 

True... If you push it and hold it for about 4 seconds, the car will turn off and you'll have to come to a complete stop to restart it. When I did it, I was going under 25 and the system did turn off. I did not push it again, as I did not want the steering system to lock.
I was interested in this after the Prius run-aways.

This below is out of the service manual and is the "notes" section of a table in the "POWER CONTROL SYSTEM" on page 38 & 39.
*1: When the ECVT selector lever position is N position, the hybrid system start condition is different according to the vehicle speed.
• At vehicle speed of 4 km/h or less, the hybrid system can start only when the brake pedal is depressed.
• At vehicle speed of 4 km/h or more, the hybrid system can start even if the brake pedal is not depressed. (It is the same as “Hybrid System stall return operation while driving”.)
*2: When the ECVT selector lever position is in any position other than P position and when the vehicle speed is 5 km/h or more, the hybrid system stop condition is different.
• Press and hold the push-button ignition switch for 2 seconds or more. (When the push-button ignition switch is pressed for too short a time, the operation may be invalid, so properly press and hold to prevent the incorrect operation.)
• Press the push-button ignition switch 3 times or more within 1.5 seconds. (Emergency stop operation)