I've got one, although it's still in bits......
I believe it's a '94 RTS-1
Awaiting the rear triangle sand-blasted, then i can spray it and re-assemble it, possibly with a change of parts.
Anyway, here it is, in all it's naked glory.
Well, in a very short explanation, he told me that the added volume of oil would keep the shock much cooler in severe conditions and longer downhills, sparing a bit the seals, making them last longer and handling better with high pressures. It also has an air chamber on the top of the reservoir, to increase or decresase the oil pressure, turning an ordinary shock into a fully adjustable one, almost like today´s shocks (but 12 years earlier)...
It seemed kinda silly to me, but i was to much happy with the new "pro" version that i didn´t care about tech details, until a few days later, when i tought about it and tuning mods started to make sense...
Edit: Cherrybomb explained this even faster than i did...