An analogy: At an OAT of +50, a jet engine at 100% HPRPM delivers, say, 70 `bananas` of effective thrust. If we now transport that engine (same RW,AUW, surface wind and pressure etc) to an airfield with an OAT of +15, the colder, denser air will make the engine more efficient, and less HPRPM (eg approx 95%) is required to achieve the same net THRUST.