Funny you should have this on today.
I got home about 3 this afternoon with another MF, this one a 10 and it too has the vari speed.
I asked the guy I got it from what it was all about, and he said just like olcowhand. 4 gears on the shifter and 5 variable speeds per gear.
Up here it is similar to a snowmobile clutch which moves in and out to change the diameter of the drive , but on these you control it manually from the lever. Great concept.