what would cause it to increase?
The distance between the knees changes and that causes your "center" to move onto one foot or the other. The visualization enables your brain to do this in a way that serves the stroke.
Generally, when the knees move farther apart, the ground reaction force will be rotational/upward. When the knees move close together, the GRF will be linear/forward. However, this should be dictated by your natural stroke.