On the Neilpryde (www.neilpryde.com) website the first thing you see there is a picture of Kauli Seadi on a wave. The boom in that picture appears to be a bit different to most and has an unusual extension setup.

Does anyone know what it is?

I looked at the Windshack website amongst others and it mentions a boom where the boom end moves down the extension instead of the extension moving. Is that what this is?

Has anyone used one? Is there a real advantage in the stiffness of that design?