Gwen, I have struggled with my Rudy to find the right combination of hand, seat, leg to keep him from shortening and raising his very upright, kind of ewe neck. A couple years ago I attended the USDF Adult Series Clinic with HG in which she emphasized a shorter but more forward rein. It took me the better part of a year to coordinate all the parts but I think I have mastered it now. Keys are 1) keeping the elbows bent AND just in front of your body, like holding a tray or a couple glasses of wine out in front of you. 2) Sitting to the front of the saddle directly over your thighs, keeping chest open (no hunching allowed.) 3) Hands should be (at least for my horse) very quiet--no fiddling allowed. 4) Legs in contact with horse all the way down to the ankle without a gripping knee. This last one was the most difficult for me given my pony's apple shape; otoh, I know Rudy is truly "through" when my leg just naturally drapes around him all the way down. With your elbows bent, it is very easy to soften your arms and to even give the rein (like when jumping) and with your hands very quiet, your horse will hear the whispers instead of having to "shout." As Khall noted, hands too high will make curling worse and as someone else noted, hands too low will work on the bars and may cause inversion. I try to picture those BNT types when I ride in order to get all my parts in the right place. Video and pony suggest I am not always successful but it is getting better.

Y'all are absolutely welcome to chime in and agree or disagree. Hoping for a good discussion!
ETA: Even though Tio is still very green and has the perfectly shaped hunter's neck, I try to ride him the same way as Rudy when it comes to the reins. He really appreciates the quietness of body and hands and resents it if i get too fiddly. Supremely hard to do when he tries to counter bend or take off for "the hills."