This is the speed gait. The foot fall is a lateral movement. The support first by two, then by one foot hoof at a time with suspension of the other legs. It is considered a 1-2 set down gait. So there is either 3 feet in the air or 2 feet in the air while the others are on the ground. Some describe the feel of the horse's front end as climbing a ladder. The speed can be so fast as to blur the images of the legs. Some viewed by the naked eyes may look like a pace and can only be seen by using the slow motion of a camera.
The following are examples of the flat shod rack as demonstrated by the three breeds indicated.
Saddlebred
(Show Flat Shod)

High knee and hock action, a near vertical head and upright neck carriage, can overstep the front. |
Racking Horse
(Flat Shod)

Not exaggerated high action, can overstep front tracks. |
Rocky/Kentucky Mountain Horse

Medium action and length of step. |
Non-Morgan video examples:
http://www.youtube.com/watch?v=uUS-4jd4R2Y