The tie line drawn is from the solid alpha to the liquid and by dropping a vertical line down at these points the mass fraction of each phase is directly read off the graph, that is the mass fraction in the x axis element.
In this context, a line is drawn to connect the solid alpha with the liquid, and by drawing a vertical line downwards at those specific points, the percentage of each phase present in the mixture can be readily identified from the graph, specifically within the x-axis section.