There was a thread a few months back about placing the thumb on the blade. If I knew how, I would link to it. When I started with a str8, I had my thumb on the blade, but after reading the previous thread I tried placing my thumb under the shank. It didn't feel as though I had enough control of the blade in the tricky areas but did make it easier to scythe the blade in the wide open areas. End result is I use both techniques, thumb on blade for short straight strokes in tight areas and thumb under shank for scything large areas. Works well for me.