kurmasana keeps the hands palm down because the work one can do with palms face up will be done in supta kurmasana. The difference you notice with palms face down is an invitation to do a different sort of work (probably in the shoulders, but that is still conjecture.)
thank you maranatha, what you’re saying makes sense to me.
and i won’t rest until i find out, then i’ll show off the answer here 
I have elevated anyone who can maintain this posture to the level closest to divinity.
could you please explain this sentence flexpenguin?
with the palms down will among other things open the shoulders out
thank you charliedharma. are the ashtangies coming out of the woodwork now ? 