The amazing Omniwheels allow it to spin circles while driving straight. When used as regular wheels they have friction. However, when sliding sideways they do not. Check out the 1:08 video on YouTube - linked below.
Yes - this is just one "wheel" as we normally call it. Original omniwheel from Sariel (yes - same one in my favourite designers links). mahjqa has replaced the gear wheels with normal LEGO tyres and wheels
More photos at mahjqa's Flickr page. Click on this link: http://www.flickr.com/photos/28134808@N02/sets/72157625017384671/with/5071779759/
Also amazing YouTube video of the Metal Grudge in action at this link: http://www.youtube.com/watch?v=fOuP_w0FZi0