What is a characteristic feature of the full Biellmann position?

The full Biellmann position is characterized by a specific and unique posture that skaters achieve while executing a spiral or an edge. In this position, one foot is lifted high and held by the hand, while the other foot remains on the ice. A key feature of the full Biellmann is indeed that both feet come into close proximity of the skater's head. This distinctive articulation not only showcases the flexibility and strength of the skater but also visually emphasizes the elegant and artistic aspects of synchronized skating routines.

The other characteristics outlined in the options do not accurately describe the full Biellmann position's requirements. For instance, while maintaining a straight back is important in many skating positions for stability and aesthetic, it is not a defining characteristic of the full Biellmann. Likewise, the arms may take various positions that enhance the flow without necessarily being parallel. Lastly, the Biellmann position relies on the entire body to maintain balance, not just the upper body. Hence, the correct choice highlights the essential aspect of foot placement that defines this advanced position in skating.