Seeing how no one is answering, I'll take a guess: On most tractors for the PTO shaft to run at 540 rpm, the motor has to run at a certain rpm; This varies from tractor to tractor and engine type, but lets say the engine has to turn 2200 rpm to make 540 Pto rpm (thus at 1100 engine= 270 pto rpm, 550 engine rpm=135 pto rpm etc...) .
Without a hydrostatic transmission you can see it becomes important to pick the right gear to determine you ground speed because you can't change your engine speed and still maintain 540 pto rpm.
What 540e does is changes this engine rpm to pto rpm ratio. So say instead of 2200 engine rpm=540 rpm, maybe now 1600engine rpm=540 pto rpm. This assumes the tractor's engine can supply the power required for the implement at 1600 rpm. If so, you can reduce fuel consumption, noise, vibration, engine wear, etc..