I think the 35 miles per hour threshold is when the three beeps happen when you are in "forced" EV mode, and it returns to normal hybrid mode. You'll need the EV button 'modification' for North American Prii in order to do this, but the only real advantage is that the acceleration can be pretty substantial without the ICE coming on when in this mode. 'Round here it cancels at 55km/h
When not using forced EV mode it can continue to use the electric motors exlusively for longer, but the ICE will spin without using gas after 35 so that MG1 doesn't overspin. I think this is what people call 'warp stealth', but I may be mistaken.