This mechanism is actually dated to the baroque era and constitutes a bit of baroque theatre -- something that was only restored a few years ago. Each day, at around 5:30pm, the baroque machinery goes into action, the painting is lowered and thus is revealed the remains of St. Ignatius and the statue of St. Iganatius, entirely changing the look and feel of this altarpiece.
The spectacular statue of St. Ignatius it reveals was originally designed by Pierre le Gros the Younger (1666-1719) in completed in the year 1699. I say "originally" because, regrettably, almost 100 years after its creation, in 1798, the arms, head and legs were taken off the status and melted down in order to pay war reparations to Napoleon. The missing pieces were re-created about 5 years after their removal.
A truly remarkable piece of liturgical art in its own right, and one which testifies to the beauty of other liturgical arts, in this case sacred vestment work, from the period.