This lower part of a backplate is made up of three upwardly curving lames. A lame is a small piece of metal used within a larger section of armour and helps provide movement. This would be useful on the waist to allow the knight wearing it to bend and move from the torso, while still protecting him in combat.
Eaves, Ian, Walter J. Karcheski Jr., and Thom Richardson. Armour from Rhodes in the Museum of the Order of St John, Clerkenwell. London: Museum of the Order of St John, 1985.
Karcheski Jr., Walter J., and Thom Richardson. The Medieval Armour from Rhodes. Leeds: Royal Armouries and Higgins Armory Museum, 2000. Pages xi, 58–59, 141 and 145