Interesting, it seems like how Dunipace is describing it is how I think about it. It just seems counter to seeing Eagle and other players putting the thumb more towards the center. I can't imagine how I would be able to grip the disc hard and allow it to pivot with the thumb that far inside. It must be moving back towards the rim at release or something.
Also my favorite video of Dunipace is him casually hitting two standstill 50' putts in a row. Never knew he had that level of skill.
Give yourself a fan grip then try Eagle's thumb in middle of disc. You'll feel massive leverage but I think the tricky party is keeping it nose down. Bigger hands probably help too.