Here’s some interesting footage from what appears to be an instructional film from the French Cavalry School at Saumur. The subject matter is the Passage, Piaffe, transition from the Passage to the Piaffe, and the Spanish Trot.
There is some really amazing slow-motion photography that shows the exact difference between these particular High School airs. This footage is worth a careful watching because you can clearly observe the coordination of the aids for these airs.
Watch video: Passage, Piaffe, Spanish Trot.