Dr. Heinz Meng noticed that goshawks and bow perches were not a healthy combination in terms of the wear on the tail feathers. In the mid 80's he wrote an article discussing the merits of a new perch design he had come up with that he called the "Tail Saver". He had realized that by moving the tie off point above the ground so that the angle of the leash between the hawk and the perch at the end of the bate was about 45 degrees the hawk would no longer be beating its tail on the ground.
What Dr. Meng did not realize until much later was that the concept of a high tie off point was also much safer for the legs and hips to hawks than perch methods using a low tie off point.
Several other falconers have created and used perch designs with a high tie off point, and I really don't know how to give proper credit for those discoveries. Many of them pre-date Dr. Meng, and some are likely adaptations of his ideas. Dr. Meng is the name that is commonly credited with the concept, even if it is not quite completely accurate to give him exclusive credit.
There are two critical points that can cause injury during a bate. The one that is commonly thought about is when the hawks forward momentum is halted by the leash assembly. Care is generally given to keep the leash short so that the hawk cannot build up excessive speed and to make sure the jesses are of equal length to make sure the strain is spread out equally between the hawks feet and to make the sure that the point of contact between the jesses and the feet is not a rigid edge.
Another critical point during the bate that is often overlooked is the energy recoil that can occur right at the end of the bate. Newton's third law of motion (for every action there is an equal and opposite reaction) applies to bating hawks just as it applies to everything else in the universe. This is the area of danger that the high tie off point type of perch specifically addresses. More than a few falconers refer to this as the "Meng Effect" which I feel is a suitable tip of the hat to the man who has likely spent the most time analyzing this solution to hawk husbandry.
The simplest way to really explore the intersection of Newton's third law and bating hawks is to examine high tie off perches and low tie off perches separately.
Bate from a low tie off perch
Bate from a high tie off perch
There are several things that can be done to dampen the effects of the last moments of a bate on a perch with a low tie off point. Using a leash with some shock absorbing properties helps greatly - for example braided leashes naturally stretch, as do leashes with elastic built into the design. An elastic shock absorber can also be added between the leash and the leash ring.
There are many perches designed to mechanically dissipate the energy at the end of a bate. Some examples are bow perches with nice shallow arches and/or heavy leash rings, rotating ring style perches, and many types of block perches where the ring mechanically rotates around the block. One weakness to these mechanical means of dissipating the energy of the bate is that they are largely to completely ineffective if the hawk bates more than one time in a row in the same direction.
The very simplest way to avoid these problems is to move the tie off point up off the ground.
The actual distance necessary between the ground and the tie off point to achieve positive results depends on the species of hawk being tied down. The larger the hawk, the further off the ground they need to be tied to achieve the benefit of a high tie off point. It is commonly stated that the leash needs to be at a 45 degree angle when the hawk reaches the end of the leash.
A much more critical point is that of the distance between the tie off point and the ground. If the tie point is high enough to keep the hawks legs off the ground when the leash is pulled out in a straight line, then the perch will have all the benefits of a high tie point.