This horse is not racking, it's trotting. The rider is also posting because it's trotting, at the slow gait or rack the rider would sit in the saddle because those gaits are smooth. I was going to link a picture of a horse racking that I found online but this website doesn't seem to allow links. If you go to the wikipedia page for "horse gaits" a good example of the rack is on the bottom right, just about the sources. Note that the trot above is a two beat diagonal gait, meaning the front foot moves forward in unison with the opposite back foot. A pace is a 2 beat lateral gait, mean the front and back feet move forward in unison. The rack is a 4 beat lateral gait meaning the front and back legs of each side move forward somewhat together but not in unison. In the picture on wikipedia, you can can see that the horse only has one foot on the ground and the order of the legs movement is much different than the trot.