Bike porn

Braker's got one of those on his Canyon. I saw it and asked "wow thats a strange top cap, what is it, how does it work, how do you tension it" and he just shrugged and said magnets or summink. Don't wanna disrupt pron, but how TF does it work? No bolt within. Found a better pic.

Possibly in a similar way to Easton's RAD.

http://www.eastoncycling.com/bike/wp-content/uploads/2010/04/Relief-Area-Design.pdf