Teaching a lengthening takes commitment on the horse's part. What I mean by that is, when you ask for a smaller trot, bigger canter, more active walk, etc, your horse should respond to that request immediately. Teaching transitions within the trot can be very difficult since a bigger trot requires a lot more work on the horse's part. Most try to avoid the work by jumping into the canter instead which is a much easier gait for them. If you start working on lengthenings, please make sure you first have good command of both reins and legs, your half halts are effective, and your horse is attentive to you. I spent an entire clinic working them with my 6 year old TB and was surprised to find all that was involved. Good luck.