I love "aha" moments! Now you just have to remember what you did so you can do it again.
Strawberrypoint, spiraling in and out at the trot or canter will almost always engage a horse's hind end at some point. Every horse has a natural circle where they will naturally fall into perfect balance because they will be bent properly from poll to tail. By spiraling in and out, you eventually hit the sweet spot and get those aha moments. Once you find that, and get both horse and rider used to what it feels like, spiral back out and try to keep the proper bend to do it on a larger circle and eventually on straight lines.