No one is suggesting a 90 angle in the elbow, that would likely break the line straight line from elbow to horses mouth. The use of a bucking strap is really only for work on a lunge, to stabilize the seat. If used as a way to still the hand rather than stablize the seat, the upper arm is too advanced to a hang vertically. I too use to say to do this, years ago I too though this low/fixed posture was a good idea, but it is a bandaid. And over the years I have found it then just results in the student having to relearn, which makes it a waste of time. IF the rider is so unstable they need to hold it not to fall back (very beginning rider) fine. But its worst effect of all is that the action of the seat is compromised when the rider has learn to ride with an advanced upper arm.
And yes, the shoulder socket/elbow must allow the telescoping gesture within walk/canter (or the horse will end up nodding/become hyper mobile in the head/neck or slowing/resisting).
The length of the arms has NOTHING to do with it, it is rather they necessitate a different rein length. Oftentimes the arms look short because of the breadth of the rider. The compromise of the positioning of the upper arms (forward) WILL affect whether the seat is able to be used correctly, riders who advance the upper arms often end up having to rider btv in order to have the pelvis work correctly.
The height of the hands has a direct affect upon the behavior of the horse. Hands too low act on the bars, horse hollows. Hands steadily too high the horse will curl over. Hands too narrow befoe the horse finds the center will result in lack of sustaining the connection, hands too wide are slightly unstable. That said: a straight line from elbow to horses mouth is about 6" or more, and opening rein (not sustained open, but an action) helps the horse 'find the center' (And wide hands will prevent a horse who might run out at a fence from doing so.)
Hands which pull (back) is a specific use/action/manipulation, usually from too low/fixed hands because the riders are leaning over the hands. Without a developed/aligned/independent seat (which includes a proper holding of the reins/etc) will be for naught.
I DO have a problem with "low" hands, they are very agressive esp if the horse is low/closed, worse yet when they are too wide.
Keeping the hands narrowed does not prevent them from dropping. The rider has to think they are carrying a tray with martinis, if the tray is too low or forward the drinks will spill/slide off. The mouth is that fragile. Slightly wider (not stiff) allows the horse to find contact/proper bearing the most easily in training. Of course there is always a conflict of the rider learning how to sit/have aid independent aids and that needed for methodical training. And arm/hand out in front of the rider whether low or high will NOT be as stable as a hand which closer to the trunk because of a properly carried upper arm.
Look at the riders with the most proper (upper arm) alignment: Waetjen, Podhajsky, Klimke, or the newest: Google Image Result for http://i.telegraph.co.uk/multimedia/archive/02298/horse_2298958b.jpg
or gal http://www.google.com/imgres?imgurl=...Q9QEwAg&dur=70
(Remember too that these riders have narrower hands because the horses are fully trained).
Yes, the rider has to be able to use uberstreichen (give and retake the rein), but to develop self carriage in the horse means that the horse must be 'held by the seat'.
When the upper arm is in advance of the trunk and the horse 'gives to the bit' and flexes (should be only a mobile jaw but is often more), a rider will end up moving their upper arm backward until the hand is NEUTRAL (that only occurs when the rider's arm muscles are relaxed..and that is vertical/thumbs up). Hence the horse is NOT given reward for changing its behavior
What creates a sore mouth? An unstable carriage of a busy hand/arm. Action on the bars or sawing onto the tongue? . Those is an improper use of the hand in which half halts which are the result of a fisted action rather than of the seat or a use of the wrist slightly pivoting. Again, riders are not taught the rein effects as part of training.
Pressing the hands away from the body (except to follow bascule/allow forward,down,out in 'chewing the reins from the hand) are intentional actions for purpose for TRAINING the horse.
Sitting on a horse WITHOUT use of the hand/arm (ie on a lunge) shows independence. That is why the hands are dropped straight down from shoulder is done, to educate the rider's balance/seat. And to challenge the rider, to open the shoulders, etc exercises like backstroke, swinging, etc are done.