Discussions of hybrids keep repeating claims that simply are not based in reality. An example from this review:
"I have a friend who uses her 2010 Prius to pick up her kids and groceries ... and swears she almost never pays for gasoline."
The Prius C is not a plug-in hybrid. Any energy stored in the traction battery came from burning gas, directly or indirectly.
Running in the electric-only mode does not create energy from thin air. It just means the traction battery is being depleted. And the only way to refill it, directly or "laundered" by slowing or going downhill, is by burning gas to run the ICE to speed up, go uphill or just act as a generator.
The high mileages claimed by some who maximize use of the electric mode probably are because they are driving slower on average and increasing their trip time. In fact, they may be losing mileage because of the conversion losses of excessively using the ICE to fill up the traction battery, and then converting that energy back into propulsion instead of using the ICE directly.
Definitely people new to hybrids will find they are filling up less often, and buying less gas each time. But hybrids don't violate the laws of physics.