The Prius is radical: both “rad” and iconoclastic. EV Mode stands in evidence of both. If you’re not familiar, there is a wiki on EV Mode and its button:
Everything you ever wanted to know about EV Mode, courtesy EAA-PHEV.org
But you can experience it first hand on our shop car or your own Prius, so long as it is an 04 model or newer. 
Toyota offers a stock EV button (pictured on LG’s car) available through Sigma Automotive ($235 parts and installation) particular to 04-05 and 06-08 models. Coastal Etech offers a very slick alternative using the “cancel” function of the cruise control branch. (LG’s price for the part and installation: $170.)
EV mode greatly amplifies the thrill of regular “stealth mode”, with greater power and speeds up to 34 mph, and it eliminates the anxiety of making minute trips (for street cleaning, e.g.) in the seven seconds before the ICE turns on. You will also know the hunger for longer EV performance, and therefore bigger battery capacity, of the plug-in hybrid community—those that spend long weekends in the confines of their garages troubleshooting circuit boards and investigating lithium-phosphate chemistry, in the pursuit of the coveted AER (all-electric range).
