Doesn't show well, but I think the wheels are turning rollers which in turn turn the tracks. It is difficult to really determine on video because of the stroboscope effect of the camera shutter, but if you look real careful when he's just crawling, it looks like the wheels are running forward when the track is going forward.
The units are attached by a turnbuckle to a hub on the center of the wheel, and there were chains on the inside - probably to keep the assembly on.
One advantage, is that the truck has about a foot more clearance before the bottom drags. Son found out the hard way when he tried to go down the lane the other day. When you're sitting on the drift with all four hanging off, you're in trouble. Took a pretty good effort for the old Massey to drag him back off!