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FUNCTION-
The PRIMARY difference in function is that the D ring, being flat against the horse’s cheek, is less likely to pull through the horse’s mouth when the horse is trying to evade the reins used as a turning aid. It pulls against the side of the face instead. The loose ring, on the other hand, can relatevely easily be pulled into the mouth.
Therfore, the D ring is a better bit for a green horse that does not yet fully understand “steering”, or for an older horse that choses to ignore the steering aids.
The second functional difference (and it is VERY minor) is that when you pull on the reins attached to a loose ring. it tends to rotate the bit so that the joint is pointing 180 degrees from the riens. So, if you ride on steady contact, it TENDS to keep the mouthpiece more stable in the mouth (this is the opposite of what many people say, but the physics is pretty clear).
When you pull on the reins on a D ring, it doesn’t change the orientation of the mouthpiece. So the horse has more freedom to manipulate the bit into the position it prefers. (This can be good, or bad.)
HISTORY
Historically- through the 60s and into at least the 70s, The D-ring was considered a “Race horse bit” and a “bit for a very green horse”, but NOT APPROPRIATE for the hunter show ring. The common bits for show hunters were the loose ring and the full cheek. (Like the D ring, the full cheek helps with steering, but did not have the negative “race horse” connotation of the D ring.) You did nott see D-rings in the hunter show ring (or outside course).
Look at the illustration of proper position in the hunt seat equitation rules. It shows a loose ring snaffle. Look at teh first edition of George Morris’s Hunt Seat Equitation for how to measure ring size.
Some how, more recently, D rings have become more fashionable for hunters, while loose rings are more fashionable for dressage. But no judge could claim, with a straight face, that a loose ring was “unconventional”, when it is the bit illustrated in the rule book.
However, “looking the part” is part of the “initial impression”, and subconsciously affects the judge’s expectations. But that is only going to come into effect when two rounds are effectively “tied”.
“Initial Impression” that “this is someone from a different discipline” isn’t going to negate a good round. Noor is the initial impression “this is a show hunter” going to help out a poor jump.
Personally, I prefer to use an eggbutt, and have never felt that it was “used against me”.
