Lacking Impulsion lesson, Aspen derby

This Thursday we rode with John, and while I felt a lot of improvement, I am still totally flummoxed by how to generate impulsion. I now know that it is NOT by half halting around the corners on the way to the fence - unless you have a big canter. A half halt without enough canter just tips Charlie down onto his forehand, making him strung out, and resulting in pukey fences. I also know that it not just by riding faster (same thing happens as above). It matches the feeling of a canter piroutte, or a 10 meter canter circle, but how you generate it, I don't know. I know you have to use your hands as the "stop" but your seat and legs as the engine, and rely more on your back, but I'm missing what the "go" is. Charlie saved me, however, at Aspen, by being in a mood, which meant I didn't have to worry about the go at all. While our Novice show jumping and derby courses were not perfect - it was my first time ever going novice and it felt pretty smooth and like I knew what to fix and when (for the most part). Charlie loves his job and I love Charlie for loving his job. He is a total delight to ride.
Charlie's show jumping round