I'm not sure this is what he meant, but I think he refers to the temperature of the water. If you lower the pressure, it takes a lesser temperature to get the water to start boiling.

So if you go on a mountain and put a pot with water over a fire, it will start evaporating rapidly(boiling) at a lesser temperature than 100 degrees Celsius, so he needs a more efficient way to transfer more heat into the water to get it to 100 degrees Celsius(normal boiling temperature).