Dressage seat equitation: What's that about?

I've been interested in the concept of dressage equitation classes. Lendon Gray promoted this idea and now there are medal classes and finals sponsored by the USEF/USDF. I wanted to know more about it, and the USEF has a Web page with descriptions, score sheets, and an article from Dressage Today.

What constitutes good dressage seat (according to the DR117)?

DR117 The Position and Aids of the Rider.

1. All the movements should be obtained without apparent effort of the rider. He should be well balanced with his loins and hips supple, thighs and legs steady and well stretched downward. The upper part of the body easy, free and erect with the hands low and close together without, however, touching either each other or the horse and with the thumb as the highest point; the elbows and arms close to the body enabling the rider to follow the movements of the horse smoothly and freely and to apply his aids imperceptibly. This is the only position making it possible for the rider to school his horse progressively and correctly.
   
2. Not only the aids of the hands and the legs but also of the seat are of great importance in dressage. Only the rider who understands how to contract and relax his loin muscles at the right moment is able to influence his horse correctly (compare DR102.2, DR108 and DR115.3).
   
3. Riding with both hands is obligatory at all national and International Dressage Events. However, riding with one hand is permitted in the Freestyle Tests and when leaving the arena. Individuals holding a Federation Dispensation Certificate may use bridged or special adaptive reins for use with one or no hand(s), if their physical limitations require such and the equipment is listed on the Dispensation Certificate.
   
4. The use of the voice in any way whatsoever or clicking the tongue once or repeatedly is a serious fault involving the deduction of at least 2 marks from those that would otherwise have been awarded for the movement where this occurred.

General rules
Here are some of the rules and guidelines...

• There are two age groups in junior divisions (13 and under and 14 and over). While the USEF may not have official adult equitation classes, I have seen them held at local shows.
• A class can be held in a standard dressage arena with up to six riders at a time (the class can be divided into groups), or the class may be held in a warmup ring with more horses
  
• Whips and spurs are allowed, horses must wear a snaffle bit
  
• Medium walk, trot, and canter
  
• Rider equitation (position, seat, and correctness and effectiveness of the aids as required by training and first level tests.

What takes place in the class
As a group, the judge can ask for:

• Free walk
• Transitions from one gait to the next
• Walk to halt and halt to walk
• Change of direction across the diagonal or in half circle at walk or trot

Judges may ask these tests of riders individually:

• Transitions
• Leg yield
• Changes of lead through trot
• Serpentine at the trot
• Shallow loop serpentine with counter canter

Here we see the second half of an equitation class (thanks to Youtube).