7.1.4. Flexible Takeoff Procedure
To carry out a
flexible takeoff, which is always at the discretion of the pilot, a flexible temperature has to be determined from an RTOW chart computed with no derate or an equivalent computerized system. This temperature value must then be entered in the MCDU (Multipurpose Control and Display Unit) during the takeoff preparation phase (Figure C31). At the brake release point, the thrust throttles must be pushed to the FLX position (Figure C32) as per the Standard Operating Procedure (SOP).
TOGA thrust remains available at any moment during the takeoff phase. (Q.E.D.) But, in the event of an engine failure after V1, its selection is not required.
7.2.4. Derated Takeoff Procedure
....
Important:
When a derated takeoff is carried out, TOGA thrust must never be selected until the aircraft is airborne and above the minimum flap retraction speed (“F” speed). The reason for this is that performance calculations are made for minimum control speeds, different from the ones of TOGA.
Source (already provided earlier by nitpicker330): Airbus' publication:
Getting to grips with aircraft performance. on page 89 and 92.
Well, I've learned a lot. I wasn't familiar with "derated" T.O. procedures. (Never flew A330 or 340 where derated T.O.'s are available. I also didn't know it was an option on some of the A320's)