I think the most important is the action of the legs and pelvis in this type of exercises. Transitions are the ultimate exercise for training the horse to better balance and collection. By nature, the horse is balanced toward the forehand, so if you sit on your horse and let him move on long reins, 60 percent of his total weight is on the forehand. That's his natural balance, and there is nothing wrong with that. However, if you go forward to trot or canter, the horse goes more on the forehand, and we want to avoid that.
On this website, there are good advices : http://www.equestrianandhorse.com/training/dressage-horse-training/extended-trot.html
I hope this helps, bye