Hmmm,
Reply quality not too good so far
(not that mine will be any better)
Most (if not all) jet engines are flat rated. This means that the makers have guaranteed that they will produce at least a certain minimum thrust up to a certain temperature at full throttle positions.
Above this temperature (which is around 22 degs C give or take a few degrees) the rated thrust will reduce (less dense air less available thrust).
When using an assumed temperature for T/O, a temperature above the flat rated temperature is selected which is input into the FMC/TMC (the temperature selected is based on a/c weight, available runway, actual OAT, etc) This fools the computers into thinking the engine cannot produce max rated thrust due to high OAT and instead uses the rated thrust for the temperature selected. This is then used for performance calculations and autothrottle thrust settings. When the throttles are advanced and A/T selected (push TOGA) the levers will advance to the level where the reduced thrust setting is established and be held there until another thrust mode is selected.
The reason for this is to reduce engine wear and so increase engine life. In the event that there is an engine failure or obstacle clearance is required the throttle levers can be manually advanced (firewalled) to acheive full rated thrust.
Hope this helps.