I'm not 100% sure either.
But I think it's when you replace the keepers in between the gears with heavy duty snap rings.
I don't know how much you really need it or how much it really helps, I just heard someone mention it to me before...
That is a good new information for me.
No, Ive not installed the heavy duty snap rings.
Only a new bearing, bigger output shaft and some shift shafts.
Hope, I can reassemble the engine this weekend.
Im late
Mike