what is an override transmission?

It's different gears in the transmission to achieve a higher overall top speed.
It's like putting a smaller rear sprocket on to a certain extent.
 
AN OVERRIDE TRANS HAS CUT GEARS TO ENABLE YOU TO QUICKLY SHIFT THRUOGH THE GEARS AT WIDE OPEN THROTTLE,WITHOUT THE CLUTCH. ONE DRAW BACK IS, YOU CAN'T DOWN SHIFT.