The Rolls Royce Dart's on the HS748's injected a watermeths mix to restore maximum power if hot or high.
If required the pumps were switched on and the water meths kicked in at something like 3/4's max rpm.
If I remember correctly the minimum quantity in each of the 2 tanks had to be 10 gallons if the system was to be used, and roughly 1/2 of this would be used on a takeoff or go around.
'Was funny to watch as it looked like a steam train hurtling along the runway at times!
Cheers.