Well, I'm not one of the great and wise, but maybe I can shed a little light on the grey mystery pumpkin anyway.
It's easier to think of the T20 as two transmissions on a common input shaft, which is how it acts in service. There are two drums on each side with a set of planetary gears between the inner and outer on each side. In forward, the inner drum is the "drive" drum and outer is the "brake" drum, so when the sticks are pushed forward the band on the inner drum relaxes and the the outer band tightens, forcing the power from the input shaft to turn the inner drum and the drive sprocket on that side. When you pull the stick back on that side, the band on the outer drum relaxes and the inner band tightens on the drum to stop the drum, sprocket, and wheels on that side.
When it's shifted into reverse, the drums change roles and the inner becomes the "brake" and the outer; the "drive". When you pull the stick back, the band on the inner drum tightens and the band on the outer relaxes so that the power from the sun gear on the input shaft is directed through the planetary gears and drives the sprocket backwards.
This may not be 100% accurate, but it's the way things look to me.
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far