The dollies are typically pinned in two or three places to the vehicle and do not swivel relative to the tow vehicle, but they swing left or right in a turn requiring the dolly axle to pivot. Because of this pivot, most use some type of spring or dampener to keep the axle tracking true.
I have looked at these dollies a number of times and just cannot get over the price even though the engineering is sound. It would make a good companion to many truck camper combinations where vehicle payload is already used up but you still want to tow something.
These dollies were more common when people used to tow with sedans:
http://www.airforums.com/forums/f464/slimp-dolly-44260.html