Denti - 75% N1?
Lowest values I can recall seeing is approx. 85 N1.
KORD, rwy 28, 0C, 13000', 667' elevation, 120K 737, N1 is 82.6%.
To the original question - several excellent answers. Basically increase your speed with two engines running prior to a higher V1, and use that increased speed to allow better climb performance over terrain on the departure path.
Used when you have long runways(to allow increased V1 adjustment) with -
1. terrain clearance issues after departure
or
2. higher altitude airports
(or both!!!)
Net gain is increased takeoff weights.