If you read into it a bit more the D-drive is not as useful as it first appears. It requires two inputs, and if input 2 is spinning faster than input 1, then it goes forward. If input 2 is slower than input 1, then it spins backwards. The larger the difference, the faster the output spins.
He calls one of the inputs the 'control' which implies that you could just use a small variable speed electric motor to control the speed, but that does not work because the control motor needs just as much torque as the input motor.
So you need some way of applying variable speeds to the inputs to get a variable output...
It's all in the engineering report on the transmissions website. I don't have the link but it's on Google.