The locknut on the bolt clamps the hitch adapter between the two towbar bushings. The torque on the hitch adapter isn't excessive (<100 in-lbs when the Xtracycle is loaded to its 50# rear weight limit), so with the locknut torqued down the adapter won't rotate around the horizontal bolt.

The adapter might need to be repositioned and the locknut re-tightened occasionally due to creep, but it shouldn't be necessary to do this frequently.

You ask a good question. I was concerned about this, too, when designing it, and was the main reason I wanted to test it before we shipped.

-Jim
Bikes At Work
www.bikesatwork.com