Actually, DR117.7 does define the correct diagonal in rising trot.
- When rising trot is permitted in a test or class, the rider should change the diagonal when changing directions, except during a lengthening. The correct diagonal is considered to be when the rider is sitting when the outside front foot and inside hind foot are on the ground. In general, rising on the outside diagonal correctly influences the horse’s balance in movements other than straight lines. A change of direction in rising trot implies a change of diag- onal, but it is up to the rider to determine where that change should occur. If no change is made, there is no error and no deduction is made unless the balance of the horse is adversely affected, as it might be, for example, in a turn, circle or leg yield. In that case, the movement is judged accordingly and the collective marks for “Submission (Willing Cooperation)” and “Effective Use of the Aids” could be negatively impacted.