What does this figure imply about the luminosity of pre-main-sequence stars?
It shows that the mass of the (proto)star is the major factor determining bolometric magnitude. 

The brightness of the proto-star increases a little as it contracts and becomes significantly hotter (and the radius decreases), The star moves toward the left of the diagram. As fusion in the core becomes significant the star reaches the maximum point on the curve, this stabilises the core and the star settles into the main sequence, it cools slightly and becomes slightly less bright.