The working trot rising (also called the posting trot) is a two-beat gait, with a speed of approximately eight miles per hour, or 704 feet per minute. The feet strike the ground in diagonal pairs at the trot, with the right hind and left fore striking together and the left hind and right fore striking together. Your hands remain steady because the alternating diagonal pairs of feet provide stability for the animal as it moves.
When the horse is moving clockwise at the posting trot, rise when its left foreleg goes forward and sit when it strikes the ground. Moving counterclockwise, rise when the horse's right foreleg goes forward and sit when it strikes the ground.
By freeing the horse's outside foreleg, posting allows the animal to remain balanced on turns. If you rise and sit in the incorrect rhythm, you are "posting on the wrong diagonal." On corners, this error causes your weight to land on the horse's back just as the animal is reaching with its outside foreleg, which must cover more ground each step than the inside foreleg does on a turn. The result is loss of balance for the horse, creating an awkward feeling for you and often making the horse appear lame.
At the working trot rising, close your hip angle forward to allow your torso to follow the horizontal motion of the horse. When the horse is in a medium frame, your upper body should be inclined about 20 degrees in front of the vertical at the posting trot. The degree of inclination will vary somewhat if you change the horse's frame. For example, if a horse is put into a shorter frame for upper-level flatwork exercises, your upper body should reflect the vertical channeling of energy by straightening to a slightly more vertical position; and if the horse is travelling in a longer frame, your upper body should be inclined forward a little more.