Slashing the skin of a loaf creates a weak area. When the oven spring occurs the dough will expand through that weak spot. Expansion will be limited in the stronger, unslashed areas.
Conceptually this is fairly simple. Leave the skin unslashed in the direction you don't want the bread to expand. In the case of a checker slash on the surface of a boule- the edge stays stronger than the entire center so the expansion is forced upwards.
In practice, with different loaves, it may be difficult to predict exactly how the physics will occur. You can start with traditional slash patterns that you see on professionally produced bread.
For example, on long loaves, such as baguettes, the slashes are short, at a slight angle to the length, and overlapping to promote lengthening of the loaf. Whereas on wider, football-shaped loaves the slash may run parallel to the loaf to widen it a little.
As a demonstration I baked two loaves this weekend. They were identically formed as small football-shaped loaves (batards). One I slashed at an angle, the other parallel with the length.