Complex sentence: Ottavio is shown as tall and muscular, about to kneel to kiss the Pope's feet, a contemporary manner of greeting a pope: a guest would make three short bows followed by the kissing of the papal feet.
Split into simpler sentences: Ottavio is shown as tall and muscular, about to kneel to kiss the Pope's feet. This gesture was then the formal way of greeting a pope: the guest would make three short bows followed by the kissing of the papal feet.