According to Dave Kirk (who also designed the Serotta DKS rear suspension) the rear stays will offer a small amount of vertical movement to keep the rear wheel on the ground over irregular road surfaces. See his website info here: http://www.kirkframeworks.com/news.htm I've not got that many miles on the bike yet, but what I can say is that it is much 'smoother' than it was in it's orginal configuration. Far less road chatter on chip seal roads, and sharp bumps like frost heaves, feel more muted than with the straight chainstays. The bike in it's original configuration is here: http://www.kirkframeworks.com/PhotoGalleryowners.htm# Mine is the first bike.
Dave, was I reading Kirk's website right? Your bike originally was a yellow frame and had canti mounts, but you sent it back, they changed the rear triangle to the Terraplane, swapped your fork, and repainted? Nice work! And a helluva nice looking bike!