It definitely sounds simple. I wish it was. Speedy and I have been working on it for a while. What I've found is that it doesn't happen if your horse is on the forehand, if your horse is using his momentum to drive the canter rather than using rear propulsion, or if you can't keep your horse's haunches under control.
Ask me how I know.
You get the idea. A bazillion half halts will get Speedy off of his forehand and driving from behind.
That wonderful turn on the forehand exercise, half pass style, is getting Speedy's haunches in line. He isn't thrilled about it, which tells me I am on the right track, but it sure is effective.
During a ride the other day, I finally made a connection: Speedy likes to carry his haunches to the left which makes getting that walk transition relatively easy that direction. Recognizing that helped me see why we struggle getting it to the right.
Once I had that picture in my head, I half halted my sassy pony about four billion times, shoved those haunches over in a turn on the forehand another four billion times plus two, and finished it off with a fairly clean canter to walk transition.