Hey guys, I know the basics of Toque and Power and how they are related to the engine RPM. But even after a lot of googling I am unable to find an answer to this question. What is the phenomenon by which in an internal combustion engine when the torque is not enough to handle the load and no matter how much you open the throttle, the engine RPM will not go up?

I will explain my question a little bit - Basically when you twist the throttle/accelerator the carburettor MUST supply a rich fuel air mixture to the engine causing the RPM to go up. So when you are going uphill in a higher gear and the engine is unable to provide the required torque, after certain point no matter how much you open the throttle the RPM does not go up and the vehicle cannot accelerate, and at this point you decide to downshift to get more torque. So in this scenario how is it possible that opening the throttle is not causing the RPM to increase? Is it a natural phenomenon or there is some arrangement in all vehicles to cut the throttle when there is not enough torque to handle the load?