Here are my thoughts having seen bits of the presentation, from knowing what the joint looks like, and from looking at the photos:
centerlines are established on the logs
geometry developed based on the centerlines for a rafter to tie connection
(with logs of similar diameter, one could assume a near center line to center line match in how the logs plane out to one another)
the geometry of the joint in this case is developing 2 planes as primary to layout, and an interior rectilinear block of timber
the elliptical cut outs are simply a convenient way to trace the planes onto the surface of the logs - i would guess one could have several of these for various angles and log sizes - but if the cutout is close in size, flat to itself, with the angle held correctly, and the pencil used judiciously, it would seem you could make this work with a few templates - longer ellipses for shallow angles, shorter for steeper - and i'd imagine a scenario where having a bubble gauge or inclinometer mounted to the patterns would be useful
once the planes are developed, and the angles off of centerline known - the geometry can be added to both timbers
the rafter is easier to cut - in that the 2 planes can be cut completely through, and any interior joinery removed later
the tie is more difficult - as the width of the interior rectilinear shape needs to be projected onto the curved surface of the log before cutting so that one maintains the internal shape
i see timberwrestler in a few of those photos - i'll see if he can comment. and i'll try and get robert chambers to check in here. his website is here - there might be some useful information there:
http://www.logbuilding.org/TOC.htmland he makes these great scribers (i have no need for them - but have held them at a TFG conference):
http://www.chamberslogscribers.com
