The 777 FCOM doesn't give too much away in regards of takeoff bump.
Takeoff bump is a thrust reference mode which allows additional takeoff thrust when the aircraft is operated in the following envelope.
Pressure altitude -2000ft to 3000ft. Ambient temperature 32 to 53C.
Even though takeoff bump hasn't been selected but you are in the above envelope, pushing the thrust levers to max will put you into the takeoff bump increased thrust.
We only use takeoff bump after other methods have failed to produce the required TOW.
Pack off takeoff, APU to L pack takeoff.
On our Boeing fleet we don't use derated thrust for takeoff.
Oz