seal is reliant on the canals, and actually the canals play the most critical role here since the part beyond 1st bend is more elastic than the aperture of the canals. http://www.audiologyonline.com/articles/ear-impressions-art-or-science-1186
I'm not entirely sure about FitEar's logic for taking closed jaw impressions with jaw movement during the process. It's more of a school of thought in the industry than an actual fact that has been confirmed with research that which one is better than the other. Apparently those who prefer jaw movement believe that with jaw moving around high viscosity impression material will flow better and more thoroughly inside the canals, ensuring the best replica of the outer ear, though I doubt that's the case with FitEar.