Yes, it is an auxillary tank drive system. It gets deployed (lowered to the ground and actually raises the robot off its wheels) when we want to push hard. I am about to tell you more than you probably care to know about it.
It is geared down to the point where it can only move 8 inches per second max. It can produce approximately 2000 lbs of trust at stall. Needless to say, it will never stall - its not really possible to get enough traction to do that.
We would obviously only use it when we get in a pushing match when attached to a goal that is lifted enough to transfer some weight to our frame. We did not use it much in our practice competition. We only used it as an anchor so we could not be moved when we were happy with our position.
By itself it looks like a mini tank robot. There are lots of pictures of it here (its is a spare one we built):
http://www2.wildstang.com/2002/invit...2002_page2.asp
You can navigate around our website and see lots more pictures and videos.
It is very modular. We can remove it by removing 2 pins, the seatmotor flex shaft which raises and lowers it and disconnecting the motor wires. It weighs about 16 lbs. So we can remove it when we want to add some other functionality and still be within the legal weight limits.
Its mounting scheme allows it to swivel in two independent axis so that it can always maintain flat contact with the carpet.
I hope that is enough details. We will have the spare to show at the competitions we attend.