I've always had a hard time keeping them up, though. It feels so unnatural. I don't quite have literal piano hands...my hands aren't exactly horizontal. I keep them pretty much halfway between thumbs up and thumbs directly to the side, if that makes sense. It feels natural and when I try to put them up, I feel like I can't feel the horse's mouth as well and I feel like I can't make the subtle movements like I can with my thumbs turned slightly inward. It might just be in my head, though.
I looked up some videos of upper level dressage and show jumping today to make myself feel better, and I didn't see one that had their thumbs directly up. Maybe this was because of the double reins, but I'm not sure. Also, as far as the direct line goes, I don't see how it breaks the line to rotate your thumbs a little inward. If anything, it actually straightens the rein because if your thumbs are up, it means the rein is flat and horizontal to the ground at the point that it enters your hand. But to be perfectly straight, it should be horizontal to the horse and perpendicular to the ground, which is what happens when I rotate my thumbs a little inward.
I guess my question is...is it okay to ride with my thumbs rotated in a little? Is it only bad if I rotate a complete 90 degrees so my fingernails are facing downward? Does anyone else find it uncomfortable to ride with thumbs straight up?