The other half of the video isn't there... *cries*
So far what I have been able to dig up is this:
Pressure in the middle with the heel causes the horse to move it's entire body away from the pressure. Applying a slight amount of tension to the opposite rein prevents a full turn.
Yes, this is basically correct. Your oustide rein will always have some contact - no pulling, just a "handshake" contact. Ideally, you'll be riding from the seat, inside leg into outside hand to create bend and softness, but that may be a way off yet. As far as using the heel, it is ultimately a very subtle thing. Don't really dramatically turn your toes out and goose with the heel - ask first with simply more lower-calf pressure, and then move up to heel if you don't get a response. The ultimate goal is invisibility of aids, so always first ask with the softness that you eventually want to be able to use. Your seat and a shift in weight can play a great role here, too. Well-schooled horses can pick up on a weight shift alone and scoot sideways beautifully.
Pressure at the girth cues the horse to move his front end. Pressure at the flank causes him to move his rear away from the pressure.
Again, yes, but there shouldn't be a big change in your leg position. You shouldn't be reaching back, sacrificing your shoulder/hip/heel alignment, to move the hindquarters over. At girth, behind girth, and flank "buttons" are actually very
close together for most horses. If the horse is just learning, the "buttons" may be exaggerated, but a finished horse can readily discern your meaning with minimal change in leg position.
I am assuming that these types of cues are all done with the heel.
As I said above, heel as a "backup"; lower calf closure as a "true aid" so to speak.
If you think that you're riding with too much heel, thus turning your toes out too much and misaligning your legs, take a look at your riding boots, half chaps, or pants post-ride. Dirt/wear patterns will tell you where you're contact is falling and give you an idea what to change, if anything. The bulk of dirt and wear should not be on the backs of your legs.
Speed and gait are cued with the saddle and the upper leg. And there should always be contact with the rider's calf and the barrel.
Is this correct?
Yes. Dressage-speaking, the seat aids come from more than just the rider's immediate backside.
Everything from the knees to the waist can be involved in seat cues.
Lordy, this is tough to describe!!
I don't think anyone posted this link yet:The Art of Classical Riding--Dressage Training for Horse and Rider
All of the articles are worth reading, but you may find some of the "Classical Seat Series", near the top of the list, most helpful in describing the application of the aids.
Sally Swift's book Centered Riding
also discusses the aids and their application, I believe, but I don't have my copy with me right now to say absolutely for sure. Even if it doesn't, it's worth a read anyway.