This exercise is an excellent gymnastic for preparing to jump combinations. The line is complete because it includes "in and out", a vertical and an oxer, and an increasing number of strides between the jumps.
The low fences at the beginning of the grid allow the horse and rider to say calm and relaxed on the approach. Since the distances were adjusted previously, the rider can focus on his position and his body motion in order to follow his horse’s movement over the jump. He only has to be alert to keep his horse straight and balanced on his 4 feet.
To start with, keep the fence height low, especially with a young horse. The final oxer and vertical in the grid can be progressively raised, according to the level of the rider and of the horse, of course.