Horizontal vs. Vertical brushing

Horizontal brushing covers more area on your head, but it pulls all the hair in that area in the same direction. Brushing this way has the chance of creating overlapped or overlooked areas in your wave pattern. 

Overlapped Areas - when you have areas of overlapping angles, the hair gets confused because you are brushing in different directions. The photo below shows the overlapped area in pink. 

Overlooked Areas - these are the areas that your brush misses because your brush angles are too far apart. The white area in the diagram below shows the area overlooked.

Vertical brushing covers less area, and has more angles which allows for more precise brushing. This causes your waves to have a slight bend/curve to them, which looks better as they wrap around your head.