Most Chevy Volt owners understand how mountain mode functions, but in this video you'll see what happens when the operation of mountain mode is misunderstood.
Mountain mode should be activated well prior to the climb, but if you wait until the climb is upon you, then propulsion power is reduced is what you're likely to encounter if the battery pack is at or near depletion.
As the video uploader states:
"All the technology in the world is useless if this car can't climb up a hill."
With proper use of mountain mode, this car can climb a hill. RTFM, right?