density=mass / volume
when qnh decreases. pressure decreases, air density decreases.
that means for a given volume of air, the mass of air decreases. i.e. the air becomes lighter, no. of molecules decrease.
on such a day, less no. of molecules will draw less amount of fuel. so the total charge(air+fuel) into the combustion cylinder will be less. this will result in less engine output and thus the engine power decreases.
Also, less air molecules affect the aircraft aerodynamically. Lift produced is less on a day when the density is less.
Aditya