Granard: To understand , U need to know what flex thrust is.
Every rated engine has a maximum thrust, depending on construction let say 100%, to reach this thrust, U go on the upper limits (thermal and mechanical wise). A today engine is flatrated to 38deg C. That means till 38deg OAT at SLA U can get this 100% easy. Normally U don't need this thrust. To safe lifetime of the engines, U make now so called flex TO. Let's say for TO on RWY XY at AP XYZ U have 4Km . Lets say yr liftoff is after 3km, yr thust U need is 75%. Now You flex the engine: U tell the FADEC the OAT is eg. 78deg C.
The FADEC reduces the Fuel flow so, that the calculated engine limits will never reached for this OAT , U have only 75% and U safe the engine. The actual Flex temp depends on RWY lenght and temp+pressure ALT. I remember this is based on engines manufacture and flight ops from the operator. U wanna safe money U flex, wanna Fun TOGA
Hope this helps