The front drum/high gear drum has tabs (for lack of a better term) that spline into the sun shell. When the car is in first gear, the input shaft is turning clockwise (which it always does), but the sun shell and High gear drum are turning counter clockwise. When the car is shifted to second gear, the second gear servo is applied, and front band stops the high gear drum with the sun shell from spinning. When the car is shifted into third gear, the high gear clutch is engaged as the band is released which is where the 2-3 overlap comes from.
Weight saving is the benefit of the lightened one, so less reciprocating weight. Also a stock one you can sometimes see where the high gear drum tabs wear the sun shell. The aftermarket ones I haven’t seen do that. So I would argue strength is also a benefit.
