When encountering mountains, make sure you have as much charge in the batteries as possible.
For example, if you are going to be going up a mountain, use HV mode as much as possible to conserve your battery capacity.
If, for whatever reason, you've depleted your battery charge, and you know you are approaching a mountain, long-press the HV button to invoke HV-Charge mode. This will allow the car to charge the battery to a maximum of 57% (I think) so that you'll have battery power to supplement the engine/generator.
The engine in the car is a generator (for the most part) and it generates electricity to drive the car when called on.
If you're going up a mountain without much battery charge, then the engine will work very hard to generate the electricity needed to drive the electric motor in the car.
Hope that makes some sense?