Complex sentence: What makes the work unique for the period is the swoon of the Virgin Mary, collapsing in a pose that mirrors that of her son, right down to the two figures that hold her as she falls.
Split into simpler sentences: What makes the work unique for the period is the swoon of the Virgin Mary. She collapses in a pose that echos that of her son, as far as to the two figures that hold her as she falls.