What a fabulous explanation, anebel! This is a hard thing to understand - for me it came down to feeling it. When I was able to feel it, then I could train it. If you have a trainer who can talk you through it, that is great. Even better, if you can ride a trained horse who can show you, that is wonderful!
To me, it feels like harnessing energy - making it flow differently. There is a connection you establish with the horse and responsiveness to your seat. That is all I can offer, good luck!