Here is a video my friend made at our meeting of the Chesapeake Area
Metalworking Society.
It is a differential variable speed control mechanism mostly copied
from others' designs.

http://www.youtube.com/watch?v=eKzTW...ature=youtu.be

The video shows what is going on pretty well. The wheel drive part
got finished only a couple hours before I left for the meeting. It
shows better than
looking at the drive sprocket that is sort of buried. The neutral here
is a bit odd since
it is not disengaged but an infinitely low gear.

I built this from similar designs I had seen to help me understand how
the mechanism
works. The only thing I think is new is the skate wheel differential.
I had no suitable
gears for the differential and thought it would be quieter at high
speeds. Pressure
on the skate wheels is not constant. The idea was to use them as a
true disengaged
neutral but that happened at at one of the high speed ends of the
range and not near low.
Gears would carry torque better.

Differential speed controls have an extremely wide range of ratios
since they can cross zero.