i want to chime in on strategy for turtlebacks- preforms with one flatter side and one humped side. The humped side is difficult to thin. conventional wisdom says that this is a problem of centerline- it's hard to thin the humpped side because the edge is so far "above" center ( a "high" edge, when viewed with the humpped side facing down). therefore you could move the edge toward center , letting you get at the humped side. (others above suggested this) BUT- this sacrifices length and width fast. Therefore, I try to use massive percussion flakes to thin the hump with only one or two flakes which come off this "high" edge. It's kinda like a deep overshot flake. you have to set up steep strong platforms and give them a mighty whack- hard to do on smaller pieces, but i work in easy obsidian. I think this is illustrated by the pic above in timo's post where he drew a platform on that artifact photo.
to state this another way- I just ignore the rule which says platforms should be near or below the centerline. The flake wants to hinge as it dives into the hump, but with enough oomph and support, it can lead to some nice rapid thinning.
i didn't watch jackcraftys whole vid, so sorry if this is covered there...(cool technique, though !)