The A330 is a wonderfully overpowered airplane. 'Flex Thrust' is a method of tellng all those fantastic computers that the OAT is higher than it actually is. The engines then 'back off a notch' so they don't exceed EGT limits. The 'Flex Temp' is a result of all the usual perf criteria: Rwy length, airport Alt, obsatcles, TOM, QNH etc. Bottom line is, why cane the engines when you can achieve perf A performance and extend engine life and maintenance intervals etc?