According to legend, worship of the image of Madonna del Sasso dates back to the insult inflicted upon the sacred image by a soldier. After losing a significant amount of money at a game, the soldier, returning to his guard post which was near the image of the Madonna with Child, threw a stone at the Child. However, the image reflected upon itself and the Child moved onto the left arm of the Virgin. The soldier was instead swallowed by the ground which opened under his feet, as is commemorated still today by the trapdoor in the floor of the chapel near the church of St. Agostino. The displays of devotion refer to this image in numerous other painted or sculptural works in the Archdiocese as a whole.
Sources: Belli Barsali I., Guida di Lucca, Maria Pacini Fazzi Editore, Lucca, 1970 and viaggispirituali.it